izca-ru.txt

==========================================================
Обстоятельное руководство по компонентной архитектуре Zope
==========================================================

:Автор: Baiju M
:Версия: 0.5.8
:Печатный экземпляр на англ. языке: `http:https://www.lulu.com/content/1561045
                    <http:https://www.lulu.com/content/1561045>`_
:PDF-версия оригинала на англ.: `http:https://www.muthukadan.net/docs/zca.pdf
                  <http:https://www.muthukadan.net/docs/zca.pdf>`_
:Перевод: Черкашин Е.А.
:Версия перевода: 0.1


Copyright (C) 2007,2008,2009 Baiju M <baiju.m.mail AT gmail.com>.

Разрешается копировать, распространять и/или вносить изменения в этот документ
при соблюдении условий лицензии GNU Free Documentation License, версия 1.2 или
(по вашему желанию) какой-либо поздней версии, официально опубликованной Фондом
свободного программного обеспечения (Free Software Foundation).  

Исходный код программ, приведенный в этом документе, распространяется при
соблюдении положений лицензии Zope Public License, версии 2.1 (ZPL).  

ИСХСХОДНЫЙ КОД ПРОГРАММ В ЭТОМ ДОКУМЕНТЕ И САМ ДОКУМЕНТ IS PROVIDED "AS IS" AND
ANY AND ALL EXPRESS OR IMPLIED WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST
INFRINGEMENT, AND FITNESS FOR A PARTICULAR PURPOSE.  

.. sidebar:: Благодарности

  Много людей помогли мне написать эту книгу.  Первоначальный манускрипт 
  отрецензирован моим коллегой Брэдом Алленом (Brad Allen).  Когда я объявил 
  о книге в моем блоге, общественность вдохновила меня продолжить ее совершенствование.  
  Кент Тенни (Kent Tenney)  принял активное участие в редактировании книги, 
  кроме того, он переписал основной пример - приложение.  Другие присылали 
  мне замечания и исправления ошибок и недочетов, включая Лоренсо Санчеса 
  (Lorenzo Gil Sanchez), Михаэля Гаубенвалнера (Michael Haubenwallner), 
  Нандо Куинтана (Nando Quintana), Стефани Кляйн (Stephane Klein), Тима Кука 
  (Tim Cook), Камала Джилла (Kamal Gill)  и Томаса Герве (Thomas Herve).  
  Лоренсо перевел книгу на испанский язык, а Стефани на французский.  
  Огромное всем спасибо!

.. contents::
.. .. sectnum::


Приступая к работе
------------------


Введение
~~~~~~~~

Разработка больших программных систем практически всегда является сложным и
трудоемким занятием, где объектно-ориентированный подход к анализу,
проектированию и реализации (программированию) достаточно хорошо себя
зарекомендовал.  Проектирование и разработка программного обеспечения,
основывающиеся на компонентах, становятся популярными в настоящее время.  
Компонентный подход и unit-тестирование позволяет достаточно просто
разрабатывать и поддерживать программные системы.  Существует много различных
программных инфраструктур (frameworks, фреймворков), реализующих компонентное
проектирование в разных языках и средах программирования, некоторые даже не
зависят от какой-либо конкретной среды разработки.  Примерами выступают COM,
разработанный Microsoft, и XPCOM - Mozilla.  

**Компонентная архитектура Zope (Zope Component Architecture, ZCA)** - это один
из фреймворков для в среды программирования Python, реализующих компонентный
подход к проектированию и программированию.  Он позволяет разрабатывать большие
программные системы в среде программирования Python.  И делает это хорошо.  
Компонентная архитектура ZCA не требует для своего использования сервера
приложений Zope, ZCA может использоваться в разработке любого Python-приложения.  
Справедливо было бы назвать ZCA
`компонентной архитекртуой Python` (Python Component Architecture).  

ZCA позволяет использовать объекты Python эффективно.  `Компоненты` - это
объекты, которые легко использовать повторно, а к их интерфейсам можно получить
полный доступ во время исполнения программы (introspection).  `Интерфейс` - это
объект, который описывает как следует взаимодействовать с конкретной
компонентой.  Другими словами, компонента `обеспечивает сервисы` (provide),
определенные в интерфейсе.  Компонента `реализуется` (implemented) в виде класса
(class) или другого call-объекта (callable object).  И не важно, как реализована
эта компонента, а вот что важно - это то, что она соответствует `требованиям`,
определенным в интерфейсе (interface contract).  При помощи ZCA сложность системы
распределяется между множеством кооперирующихся компонент.  Механизм такой
кооперации обеспечивается двумя базовыми разновидностями компонент в ZCA:
`адаптером` (adapter) и `утилитой` (utility).  

ZCA распределен между тремя базовыми пакетами:

  - ``zope.interface``, используемый для определения (задания) интерфейсов
    компонент.  

  - ``zope.event``, обеспечивающего поддержку простого механизма инициирования и
    обработки событий.  

  - ``zope.component``, обеспечивающего создание (генерирование) и регистрацию
    компонент, а также их извлечение по запросу.  

Обращаю внимание, что ZCA сам по себе не является компонентой и не состоит из
них, ZCA скорее механизм и библиотека для создания, регистрации и обеспечения
доступа к зарегистрированным компонентам.  В продолжение к предыдущему замечанию,
`Адаптер` - это, как правило, обычный класс Python (или фабрика классов
(factory), в общем случае), а `утилита` - это обычный call-объект в среде
исполнения Python.  

Фреймворк ZCA разрабатывается как одно из направлений развития проекта Zope 3.  
Как сказано выше ZCA является чисто питоновским фреймворком, и может
использоваться в любых приложениях Python.  В настоящее время ZCA активно
используется в проектах Zope 3, Zope 2 и Grok.  Существуют и другие приложения, в
т.ч. и неинтернет приложения, использующие ZCMA [#projects]_.  

.. [#projects] http:https://wiki.zope.org/zope3/ComponentArchitecture


Краткая история проекта
~~~~~~~~~~~~~~~~~~~~~~~

Проект ZCA начался в 2001 году и был частью проекта Zope 3.  Причиной появления
проекта послужил анализ опыта, полученного при разработке комплексных
программных систем на основе Zope 2.  Джим Фултон (Jim Fulton) стал лидером
проекта.  Много разработчиков внесли свой вклад в дизайн и реализацию ZCA: Стефан
Рихтер (Stephan Richter), Филипп фон Вайтершаусен (Philipp von Weitershausen),
Гвидо ван Россум (Guido van Rossum), известный также как *Python BDFL*, Трес
Сивер (Tres Seaver), Филипп Дж Эби (Phillip J Eby) и Мартин Фаассен (and Martijn
Faassen) и др.  

В самом начале в ZCA были определены дополнительные компоненты - `сервисы`
(services) и `представления` (views), но в процессе разработки оказалось, что
`утилиты` заменяют полностью сервисы, а `мультиадаптеры` - представления.  В
настоящее время ZCA включает в себя совсем небольшое количество базовых
разновидностей компонент: `утилиты`, `адаптеры`, `подписчики` (абоненты)
(subscribers) и `обработчики` (handlers).  На самом деле `подписчики` и
`обработчики` - это два особых вида адаптеров.  

В процессе подготовки версии Zope 3.2 Джим Фултон предложил значительно
упростить фреймворк ZCA [#proposal]_.  В результате этого упрощения появился
новый общий интерфейс `IComponentRegistry`, позволяющий регистрировать как
локальные так и глобальные компоненты.  

.. [#proposal] http:https://wiki.zope.org/zope3/LocalComponentManagementSimplification

В результате от пакета ``zope.component`` перестал зависеть достаточно длинный
список других пакетов, при этом оставшиеся зависимости не относились напрямую к
разработке приложений Zope 3.  На конференции PyCon 2007 Джим Фултон предложил
включить [в пакет], используемое в пакете setuptools [фичу] `extras_require`,
чтобы дополнительно изолировать базовый набор функций ZCA от дополнительных
(add-on) [фич] [#extras]_.  

.. [#extras] http:https://peak.telecommunity.com/DevCenter/setuptools#declaring-dependencies

В марте 2009, Трес Сивер окончательно устранил зависимости пакета от
``zope.deferredimport`` и ``zope.proxy``.  

Теперь проект ZCA является независимым проектом со своим собственным планом
[релизов] и хранилищем (репозиторием) Subversion.  Проект как и прежде является
составляющей частью фреймфорка Zope [#framework]_.  При этом ошибки и недочеты,
выявляемые в ZCA, все еще  публикуются на сайте системы отслеживания ошибок
проекта Zope 3 [#bugs]_.  Кроме того, основной список рассылки `zope-dev` все еще
используется для информационного обмена разработчиками ZCA [#discussions]_.  Есть
еще другой список рассылки, предназначенный для пользователей Zope 3,
`zope3-users`, в котором можно получить ответы на вопросы относительно
фреймворка ZCA [#z3users]_.  

.. [#framework] http:https://docs.zope.org/zopeframework/
.. [#bugs] https://bugs.launchpad.net/zope3
.. [#discussions] http:https://mail.zope.org/mailman/listinfo/zope-dev
.. [#z3users] http:https://mail.zope.org/mailman/listinfo/zope3-users


Установка
~~~~~~~~~

Пакеты ``zope.component``, ``zope.interface`` и
``zope.event`` являются ядром Компонентной архитектуры Zope.  Они обеспечивают
технические средства для определения, регистрации и поиска компонент.  Пакет 
``zope.component`` и пакеты, зависящие от него, доступны в формате egg на сайте
Python Package Index (PyPI) [#pypi]_.

.. [#pypi] Repository of Python packages: http:https://pypi.python.org/pypi

Лучше всего устанавливать ``zope.component`` и его зависимые пакеты, используя
`easy_install` [#easyinstall]_ ::

  $ easy_install zope.component

.. [#easyinstall] http:https://peak.telecommunity.com/DevCenter/EasyInstall

Эта команда загружает ``zope.component`` и зависимые пакеты с сайта PyPI и
устанавливает все как библиотеки интерпретатора Python.

С другой стороны можно загрузить ``zope.component`` и зависящие от него пакеты с
сайта PyPI и, затем, самостоятельно установить их.  Установку пакетов следует
производить в порядке, описанном далее.  В операционной системе Windows
необходимо загрузить двоичные (binary) сборки следующих пакетов.

  1. ``zope.interface``
  2. ``zope.event``
  3. ``zope.component``

Для установки загруженных пакетов можно, как и раньше, использовать
команду ``easy_install``, при этом передавая в качестве параметра
устанавливаемые пакеты, аналогично двоичным egg-пакетам.  (Можно передать в
качестве параметров все пакеты одновременно в одной командной строке.)::

  $ easy_install /path/to/zope.interface-3.x.x.tar.gz
  $ easy_install /path/to/zope.event-3.x.x.tar.gz
  $ easy_install /path/to/zope.component-3.x.x.tar.gz


Кроме того, все пакеты можно разархивировать и установить по одному, например::

  $ tar zxvf /path/to/zope.interface-3.x.x.tar.gz
  $ cd zope.interface-3.x.x
  $ python setup.py build
  $ python setup.py install

Эта последовательность позволяет устанавливать ZCA как общесистемную библиотеку 
Python в директорий (папку) ``site-packages``, что, иногда, создает некоторые
проблемы.  Джим Фултон в списке рассылки Zope 3 не рекомендовал устанавливать
пакеты в общесистемной библиотеке 
Python [#systempython]_.  Рекомендуется устанавливать виртуальное окружение при
помощи программы ``virtualenv`` и/или пакета
``zc.buildout``, что создает больше возможностей для экспериментов с пакетами
Python. Кроме того, в виртуальном окружении удобно развертывать готовые
приложения (deployments).

.. [#systempython] http:https://article.gmane.org/gmane.comp.web.zope.zope3/21045


Подготовка виртуального окружения
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Существует множество популярных подходов к настройке изолированного виртуального
окружения для среды программирования Python, удобного для разработки приложений.
 Наиболее популярные — это пакет ``virtualenv``, разработанный Яном Бикингом
(Ian Biking) и 
пакет ``zc.buildout``, разработанный Джимом Фултоном.  Оба этих пакета можно
использовать одновременно.  


**virtualenv**

Пакет ``virtualenv`` устанавливается при помощи команды ``easy_install``::

  $ easy_install virtualenv

Затем создается новое окружение::

  $ virtualenv --no-site-packages myve

Команда создаст новое окружение в директории ``myve``.
Теперь в директории ``myve`` можно установить 
``zope.component`` и зависимые пакеты при помощи команды ``easy_install``, если
запустить ее в директории
``myve/bin``::

  $ cd myve
  $ ./bin/easy_install zope.component

Теперь можно импортировать пакеты ``zope.interface`` и ``zope.component`` из
интерпретатора ``python``, который находится в директории ``myve/bin``::

  $ ./bin/python

После запуска Python покажет обычное приглашение (prompt) для ввода команд и
выражений, в котором можно выполнять приводимый далее в книге программный код.


**zc.buildout**

Используя пакет ``zc.buildout`` и `рецепт` (recipe) ``zc.recipe.egg`` можно
создавать специализированные версии интерпретатора
Python с встроенными egg-пакетами.  Для этого необходимо установить пакет
``zc.buildout`` при помощи команды ``easy_install``, что также можно сделать
внутри виртуального окружения.  

Чтобы создать новое buildout-окружение для проведения экспериментов с
egg-пакетами Python, сначала создайте для него новый директорий, и, затем,
создайте в нем собственно buildout-окружение его при помощи команды 
``buildout init``::

  $ mkdir mybuildout
  $ cd mybuildout
  $ buildout init

Теперь директорий ``mybuildout`` новое buildout-окружение.  Настройки окружения
по умолчанию хранятся в файле `buildout.cfg` .  В самом начале
в нем будет содержаться следующий текст::

  [buildout]
  parts =

Надо внести в настройки следующие изменения::

  [buildout]
  parts = py

  [py]
  recipe = zc.recipe.egg
  interpreter = python
  eggs = zope.component

Запуск команды ``buildout`` из директория ``mybuildout/bin`` (без аргументов)
создаст новый интерпретатор Python в директории
``mybuildout/bin``::

  $ ./bin/buildout
  $ ./bin/python

Последняя команда создаст приглашение Python, в котором можно запускать
приводимый далее в книге программный код.


Пример задачи
-------------


Введение
~~~~~~~~

Рассмотрим приложение, предназначенное для регистрации постояльцев в гостинице.
В среде Python его можно реализовать множеством способов.  
Сначала кратко рассмотрим процедурный подход, затем перейдем к
объектно-ориентированному программированию.  После анализа результатов дизайна и
реализации задачи при помощи объектно-ориентированного подхода, рассмотрим как
данная задача решается при помощи классических шаблонов проектирования
(паттернов проектирования, design patterns) `адаптер` и `интерфейс`.  Таким
образом появится начальное представление о возможностях Компонентой архитектуры
Zope.


Процедурный подход
~~~~~~~~~~~~~~~~~~

Система храненияя данных - очень важный момент в рпзработке приложений.  
Будем использовать в рассматриваемом примере словарь (dictionary) языка Python в 
качестве такой системы хранения данных.  
Каждой записи соответствует сгенерированный уникальный ключ (ID).
Ключ ассоциируется со значением, которое будет словарем, содержащим
данные о бронировании комнаты в гостинице.  

  >>> bookings_db = {} #key - уникальный идентификатор (ID), value - данные брони

В реализации системы будет, как минимум, функция, которой передаются
данные о бронировании, и функция, которая генерирует уникальное
значение ID ключа для ассоциации с данными брони.

Генерировать значения ключа можно следующим образом::

  >>> def get_next_id():
  ...     db_keys = bookings_db.keys()
  ...     if db_keys == []:
  ...         next_id = 1
  ...     else:
  ...         next_id = max(db_keys) + 1
  ...     return next_id

Как видно из программы, реализация функции `get_next_id` достаточно простая.  
Функция получает из словаря список ключей. Если список пуст, то это значит, 
что произошло первое бронирование комнаты в гостинице,
функция возвращает `1`.  Если список не пуст, то добавить `1` к 
максмальному значению ключей и вернуть полученное значение.

Теперь созданную выше функцию встроим в программы код, создающий
записи в словаре bookings_db::

  >>> def book_room(name, place):
  ...     next_id = get_next_id()
  ...     bookings_db[next_id] = {
  ...     'name': name,
  ...     'room': place
  ...     }

В хранилище необходимо учесть, что может потребоваться хранить 
другие данные, такие как нижеперечисленные:

  - номера телефонов,
  - особенности бронирования,
  - способы платежей клиентов,
  - ...

Необходимо реализовать следующие функции:

  - отмена бронирования,
  - уточнение бронирования,
  - оплата проживания,
  - постоянное хранение данных и доступ к ним,
  - обеспечение хащиты информации в хранилище,
  - ...

Процедурная реализация данного примера требует создания большого 
числа функций, обменивающихся информацией друг с другом.  Изменения
требований к дизайну и реализации, добавление новых функций, создает 
условия к усложнению процесса поддержки программного кода.  
Кроме того, становится сложнее выявлять и исправлять ошибки.  

На этом закончим обсуждение процедурного подхода. Обеспечивать 
постоянное хранение данных, гибкость дизайна и 
возможность тестирования программного кода, будет проще, 
используя объекты.


Объектно-ориентированный подход
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. ??? should this paragraph talk about "creating an object for
 handling registration" or "creating a class to handle registration"?

Our discussion of object oriented design will introduce the `class` which
serves to encapsulate the data, and the code to manage it.

Our main class will be the ``FrontDesk``.  ``FrontDesk``, or other
classes it delegates to, will know how to manage the data for the
hotel.  We will create `instances` of ``FrontDesk`` to apply this
knowledge to the business of running a hotel.

Experience has shown that by consolidating the code and data
requirements via objects, we will end up with a design which is
easier to understand, test, and change.

Lets look at the implementation details of a ``FrontDesk`` class::

  >>> class FrontDesk(object):
  ...
  ...     def book_room(self, name, place):
  ...         next_id = get_next_id()
  ...         bookings_db[next_id] = {
  ...         'name': name,
  ...         'place': place
  ...         }

In this implementation, the `frontdesk` object (an instance of
`FrontDesk` class) is able to handle the bookings.  We can
use it like this::

  >>> frontdesk = FrontDesk()
  >>> frontdesk.book_room("Jack", "Bangalore")

Any real project will involve changing requirements.  In this
case management has decided that each guest must provide a phone
number, so we must change the code.

We can achieve this requirement by adding one argument to the
`book_room` method which will be added to the dictionary of values::

  >>> class FrontDesk(object):
  ...
  ...     def book_room(self, name, place, phone):
  ...         next_id = get_next_id()
  ...         bookings_db[next_id] = {
  ...         'name': name,
  ...         'place': place,
  ...         'phone': phone
  ...         }

In addition to migrating the data to new schema, we now have to
change all calls to ``FrontDesk``.  If we abstract the details of
guest into an object and use it for registration, the code changes
can be minimized.  We now can make changes to the details of the
guest object and the calls to ``FrontDesk`` won't need to change.

Now we have::

  >>> class FrontDesk(object):
  ...
  ...     def book_room(self, guest):
  ...         next_id = get_next_id()
  ...         bookings_db[next_id] = {
  ...         'name': guest.name,
  ...         'place': guest.place,
  ...         'phone': guest.phone
  ...         }

We still will have to change code to respond to changing requirements.
This is unavoidable, however, our goal is to minimize those changes,
thereby increasing maintainability.

.. note::

  When coding, it is important to feel free to make changes without
  fear of breaking the application.  The way to get the immediate
  feedback required is via automated testing.  With well written
  tests (and good version control) you can make changes large or
  small with impunity.  A good source of information about this
  programming philosophy is the book `Extreme Programming Explained`
  by Kent Beck.

By introducing the guest object, you saved some typing.  More
importantly, the abstraction provided by the guest object made the
system simpler and more understandable.  As a result, the code is
easier to restructure and maintain.


The adapter pattern
~~~~~~~~~~~~~~~~~~~

In a real application, the frontdesk object will need to handle
functionalities such as cancellations and updates.  In the current
design, we will need to pass the guest object to frontdesk every time
we call methods such as `cancel_booking` and `update_booking`.

We can avoid this requirement if we pass the guest object to
FrontDesk.__init__(), making it an attribute of the instance.

  >>> class FrontDeskNG(object):
  ...
  ...     def __init__(self, guest):
  ...         self.guest = guest
  ...
  ...     def book_room(self):
  ...         guest = self.guest
  ...         next_id = get_next_id()
  ...         bookings_db[next_id] = {
  ...         'name': guest.name,
  ...         'place': guest.place,
  ...         'phone': guest.phone
  ...         }
  ...
  ...     def cancel_booking(self):
  ...         guest = self.guest
  ...         #code for cancellations goes here ...
  ...
  ...     def update_booking(self):
  ...         guest = self.guest
  ...         #code for updatiion goes here ...


.. include this bit at the front of the `Adapters` section when I get
    the equivalent quote from the Patterns book to start the 
    `Interfaces` section

    The solution we have reached is a common design pattern called,
    `Adapter`.  The `Gang of Four` [#patternbook]_ give this as the
    *intent* of Adapter::

     "Convert the interface of a class into another interface clients
     expect.  Adapter lets classes work together that couldn't otherwise
     because of incompatible interfaces."

The solution we have reached is a well known pattern, the *adapter*.
In general, an adapter *contains* an *adaptee*::

  >>> class Adapter(object):
  ...
  ...     def __init__(self, adaptee):
  ...         self.adaptee = adaptee

This pattern will be useful in dealing with implementation details
which depend on considerations such as:

- changing customer requirements
- storage requirements (ZODB, RDBM, XML ...)
- output requirements (HTML, PDF, plain text ...)
- markup rendering (ReST, Markdown, Textile ...) 

ZCA uses adapters and a *component registry* to provide the capability
to change implementation details of code via *configuration*.

As we will see in the section on ZCA adapters, the ability to
configure implementation details provides useful capability:

- the ability to switch between implementations
- the ability to add implementations as needed
- increased re-use of both legacy and ZCA code

These capabilities lead to code that is flexible, scalable and
re-usable. There is a cost however, maintaining the component registry
adds a level of complexity to the application.  If an application will
never require these features, ZCA is unnecessary.

We are now ready to begin our study of the Zope Component
Architecture, beginning with interfaces.


Interfaces
----------

Introduction
~~~~~~~~~~~~

The README.txt [#readmes]_ in path/to/zope/interface defines
interfaces like this::

    Interfaces are objects that specify (document) the external behavior
    of objects that "provide" them.  An interface specifies behavior
    through:

    - Informal documentation in a doc string

    - Attribute definitions

    - Invariants, which are conditions that must hold for objects that
      provide the interface

The classic software engineering book `Design Patterns` [#patternbook]_
by the `Gang of Four` recommends that you "Program to an interface,
not an implementation".  Defining a formal interface is helpful in
understanding a system.  Moreover, interfaces bring to you all the
benefits of ZCA.

.. [#readmes] The Zope code tree is full of README.txt files which
    offer wonderful documentation.
.. [#patternbook] http:https://en.wikipedia.org/wiki/Design_Patterns

An interface specifies the characteristics of an object, it's
behaviour, it's capabilities.  The interface describes *what* an
object can do, to learn *how*, you must look at the implementation.

Commonly used metaphors for interfaces are `contract` or `blueprint`,
the legal and architectural terms for a set of specifications.

In some modern programming languages: Java, C#, VB.NET etc, interfaces
are an explicit aspect of the language.  Since Python lacks
interfaces, ZCA implements them as a meta-class to inherit from.

Here is a classic *hello world* style example::

  >>> class Host(object):
  ...
  ...     def goodmorning(self, name):
  ...         """Say good morning to guests"""
  ...
  ...         return "Good morning, %s!" % name

In the above class, you defined a `goodmorning` method.  If you call
the `goodmorning` method from an object created using this class, it
will return `Good morning, ...!` ::

  >>> host = Host()
  >>> host.goodmorning('Jack')
  'Good morning, Jack!'

Here ``host`` is the actual object your code uses.  If you want to
examine implementation details you need to access the class ``Host``,
either via the source code or an API [#api]_ documentation tool.

.. [#api] http:https://en.wikipedia.org/wiki/Application_programming_interface

Now we will begin to use the ZCA interfaces.  For the class given
above you can specify the interface like this::

  >>> from zope.interface import Interface

  >>> class IHost(Interface):
  ...
  ...     def goodmorning(guest):
  ...         """Say good morning to guest"""

As you can see, the interface inherits from zope.interface.Interface.
This use (abuse?) of Python's class statement is how ZCA defines an
interface.  The ``I`` prefix for the interface name is a useful
convention.


Declaring interfaces
~~~~~~~~~~~~~~~~~~~~

You have already seen how to declare an interface using
``zope.interface`` in previous section.  This section will explain the
concepts in detail.

Consider this example interface::

  >>> from zope.interface import Interface
  >>> from zope.interface import Attribute

  >>> class IHost(Interface):
  ...     """A host object"""
  ...
  ...     name = Attribute("""Name of host""")
  ...
  ...     def goodmorning(guest):
  ...         """Say good morning to guest"""

The interface, ``IHost`` has two attributes, ``name`` and
``goodmorning``.  Recall that, at least in Python, methods are also
attributes of classes.  The ``name`` attribute is defined using
``zope.interface.Attribute`` class.  When you add the attribute
``name`` to the ``IHost`` interface, you don't set an initial value.
The purpose of defining the attribute ``name`` here is merely to
indicate that any implementation of this interface will feature an
attribute named ``name``.  In this case, you don't even say what type
of attribute it has to be!.  You can pass a documentation string as a
first argument to ``Attribute``.

The other attribute, ``goodmorning`` is a method defined using a
function definition.  Note that `self` is not required in interfaces,
because `self` is an implementation detail of class.  For example, a
module can implement this interface.  If a module implement this
interface, there will be a ``name`` attribute and ``goodmorning``
function defined.  And the ``goodmorning`` function will accept one
argument.

Now you will see how to connect `interface-class-object`.  So object
is the real living thing, objects are instances of classes.  And
interface is the actual definition of the object, so classes are just
the implementation details.  This is why you should program to an
interface and not to an implementation.

Now you should familiarize two more terms to understand other
concepts.  First one is `provide` and the other one is `implement`.
Object provides interfaces and classes implement interfaces.  In other
words, objects provide interfaces that their classes implement.  In
the above example ``host`` (object) provides ``IHost`` (interface) and
``Host`` (class) implement ``IHost`` (interface).  One object can
provide more than one interface also one class can implement more than
one interface.  Objects can also provide interfaces directly, in
addition to what their classes implement.

.. note::

  Classes are the implementation details of objects.  In Python,
  classes are callable objects, so why other callable objects can't
  implement an interface.  Yes, it is possible.  For any `callable
  object` you can declare that it produces objects that provide some
  interfaces by saying that the `callable object` implements the
  interfaces.  The `callable objects` are generally called as
  `factories`.  Since functions are callable objects, a function can
  be an `implementer` of an interface.


Implementing interfaces
~~~~~~~~~~~~~~~~~~~~~~~

To declare a class implements a particular interface, use the function
``zope.interface.implements`` in the class statement.

Consider this example, here ``Host`` implements ``IHost``::

  >>> from zope.interface import implements

  >>> class Host(object):
  ...
  ...     implements(IHost)
  ...
  ...     name = u''
  ...
  ...     def goodmorning(self, guest):
  ...         """Say good morning to guest"""
  ...
  ...         return "Good morning, %s!" % guest

.. note::

  If you wonder how ``implements`` function works, refer the blog post
  by James Henstridge
  (http:https://blogs.gnome.org/jamesh/2005/09/08/python-class-advisors/) .
  In the adapter section, you will see an ``adapts`` function, it is
  also working similarly.

Since ``Host`` implements ``IHost``, instances of ``Host`` provides
``IHost``.  There are some utility methods to introspect the
declarations.  The declaration can write outside the class also.  If
you don't write ``interface.implements(IHost)`` in the above example,
then after defining the class statement, you can write like this::

  >>> from zope.interface import classImplements
  >>> classImplements(Host, IHost)


Example revisited
~~~~~~~~~~~~~~~~~

Now, return to the example application.  Here you will see how to
define the interface of the frontdesk object::

  >>> from zope.interface import Interface

  >>> class IDesk(Interface):
  ...     """A frontdesk will register object's details"""
  ...
  ...     def register():
  ...         """Register object's details"""
  ...

Here, first you imported ``Interface`` class from ``zope.interface``
module.  If you define a subclass of this ``Interface`` class, it
will be an interface from Zope Component Architecture point of view.
An interface can be implemented, as you have already seen, in a class
or any other callable object.

The frontdesk interface defined here is ``IDesk``.  The documentation
string for interface gives an idea about the object.  By defining a
method in the interface, you made a contract for the component, that
there will be a method with same name available.  For the method
definition interface, the first argument should not be `self`,
because an interface will never be instantiated nor will its methods
ever be called.  Instead, the interface class merely documents what
methods and attributes should appear in any normal class that claims
to implement it, and the `self` parameter is an implementation detail
which doesn't need to be documented.

As you know, an interface can also specify normal attributes::

  >>> from zope.interface import Interface
  >>> from zope.interface import Attribute

  >>> class IGuest(Interface):
  ...
  ...     name = Attribute("Name of guest")
  ...     place = Attribute("Place of guest")

In this interface, guest object has two attributes specified with
documentation.  An interface can also specify both attributes and
methods together.  An interface can be implemented in a class, module
or any other objects.  For example a function can dynamically create
the component and return, in this case the function is an implementer
for the interface.

Now you know what is an interface and how to define and use it.  In
the next chapter you can see how an interface is used to define an
adapter component.


Marker interfaces
~~~~~~~~~~~~~~~~~

An interface can be used to declare that a particular object belongs
to a special type.  An interface without any attribute or method is
called `marker interface`.

Here is a `marker interface`::

  >>> from zope.interface import Interface

  >>> class ISpecialGuest(Interface):
  ...     """A special guest"""


This interface can be used to declare an object is a special guest.


Invariants
~~~~~~~~~~

Sometimes you will be required to use some rule for your component
which involve one or more normal attributes.  These kind of rule is
called `invariants`.  You can use ``zope.interface.invariant`` for
setting `invariants` for your objects in their interface.

Consider a simple example, there is a `person` object.  A person
object has `name`, `email` and `phone` attributes.  How do you
implement a validation rule that says either email or phone have to
exist, but not necessarily both.

First you have to make a callable object, either a simple function or
callable instance of a class like this::

  >>> def contacts_invariant(obj):
  ...
  ...     if not (obj.email or obj.phone):
  ...         raise Exception(
  ...             "At least one contact info is required")

Then define the `person` object's interface like this.  Use the
``zope.interface.invariant`` function to set the invariant::

  >>> from zope.interface import Interface
  >>> from zope.interface import Attribute
  >>> from zope.interface import invariant

  >>> class IPerson(Interface):
  ...
  ...     name = Attribute("Name")
  ...     email = Attribute("Email Address")
  ...     phone = Attribute("Phone Number")
  ...
  ...     invariant(contacts_invariant)

Now use `validateInvariants` method of the interface to validate::

  >>> from zope.interface import implements

  >>> class Person(object):
  ...     implements(IPerson)
  ...
  ...     name = None
  ...     email = None
  ...     phone = None

  >>> jack = Person()
  >>> jack.email = u"jack@some.address.com"
  >>> IPerson.validateInvariants(jack)
  >>> jill = Person()
  >>> IPerson.validateInvariants(jill)
  Traceback (most recent call last):
  ...
  Exception: At least one contact info is required

As you can see `jack` object validated without raising any exception.
But `jill` object didn't validated the invariant constraint, so it
raised exception.


Adapters
--------


Implementation
~~~~~~~~~~~~~~

This section will describe adapters in detail.  Zope Component
Architecture, as you noted, helps to effectively use Python objects.
Adapter components are one of the basic components used by Zope
Component Architecture for effectively using Python objects.  Adapter
components are Python objects, but with well defined interface.

To declare a class is an adapter use `adapts` function defined in
``zope.component`` package.  Here is a new `FrontDeskNG` adapter
with explicit interface declaration::

  >>> from zope.interface import implements
  >>> from zope.component import adapts

  >>> class FrontDeskNG(object):
  ...
  ...     implements(IDesk)
  ...     adapts(IGuest)
  ...
  ...     def __init__(self, guest):
  ...         self.guest = guest
  ...
  ...     def register(self):
  ...         guest = self.guest
  ...         next_id = get_next_id()
  ...         bookings_db[next_id] = {
  ...         'name': guest.name,
  ...         'place': guest.place,
  ...         'phone': guest.phone
  ...         }


What you defined here is an `adapter` for `IDesk`, which adapts
`IGuest` object.  The `IDesk` interface is implemented by
`FrontDeskNG` class.  So, an instance of this class will provide
`IDesk` interface.

::

  >>> class Guest(object):
  ...
  ...     implements(IGuest)
  ...
  ...     def __init__(self, name, place):
  ...         self.name = name
  ...         self.place = place

  >>> jack = Guest("Jack", "Bangalore")
  >>> jack_frontdesk = FrontDeskNG(jack)

  >>> IDesk.providedBy(jack_frontdesk)
  True

The `FrontDeskNG` is just one adapter you created, you can also
create other adapters which handles guest registration differently.


Registration
~~~~~~~~~~~~

To use this adapter component, you have to register this in a
component registry also known as site manager.  A site manager
normally resides in a site.  A site and site manager will be more
important when developing a Zope 3 application.  For now you only
required to bother about global site and global site manager ( or
component registry).  A global site manager will be in memory, but a
local site manager is persistent.

To register your component, first get the global site manager::

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()
  >>> gsm.registerAdapter(FrontDeskNG,
  ...                     (IGuest,), IDesk, 'ng')

To get the global site manager, you have to call
``getGlobalSiteManager`` function available in ``zope.component``
package.  In fact, the global site manager is available as an
attribute (``globalSiteManager``) of ``zope.component`` package.  So,
you can directly use ``zope.component.globalSiteManager`` attribute.
To register the adapter in component, as you can see above, use
``registerAdapter`` method of component registry.  The first argument
should be your adapter class/factory.  The second argument is a tuple
of `adaptee` objects, i.e, the object which you are adapting.  In this
example, you are adapting only `IGuest` object.  The third argument is
the interface implemented by the adapter component.  The fourth
argument is optional, that is the name of the particular adapter.
Since you gave a name for this adapter, this is a `named adapter`.  If
name is not given, it will default to an empty string ('').

In the above registration, you have given the adaptee interface and
interface to be provided by the adapter.  Since you have already given
these details in adapter implementation, it is not required to specify
again.  In fact, you could have done the registration like this::

  >>> gsm.registerAdapter(FrontDeskNG, name='ng')

There are some old API to do the registration, which you should avoid.
The old API functions starts with `provide`, eg: ``provideAdapter``,
``provideUtility`` etc.  While developing a Zope 3 application you can
use Zope configuration markup language (ZCML) for registration of
components.  In Zope 3, local components (persistent components) can
be registered from Zope Management Interface (ZMI) or you can do it
programmatically also.

You registered `FrontDeskNG` with a name `ng`.  Similarly you can
register other adapters with different names.  If a component is
registered without name, it will default to an empty string.

.. note::

  Local components are persistent components but global components are
  in memory.  Global components will be registered based on the
  configuration of application.  Local components are taken to memory
  from database while starting the application.


Querying adapter
~~~~~~~~~~~~~~~~

Retrieving registered components from component registry is achieved
through two functions available in ``zope.component`` package.  One of
them is ``getAdapter`` and the other is ``queryAdapter`` .  Both
functions accepts same arguments.  The ``getAdapter`` will raise
``ComponentLookupError`` if component lookup fails on the other hand
``queryAdapter`` will return `None`.

You can import the methods like this::

  >>> from zope.component import getAdapter
  >>> from zope.component import queryAdapter

In the previous section you have registered a component for guest
object (adaptee) which provides `IDesk` interface with name as
'ng'.  In the first section of this chapter, you have created a guest
object named `jack` .

This is how you can retrieve a component which adapts the interface of
`jack` object (`IGuest`) and provides `IDesk` interface also with
name as 'ng'.  Here both ``getAdapter`` and ``queryAdapter`` works
similarly::

  >>> getAdapter(jack, IDesk, 'ng') #doctest: +ELLIPSIS
  <FrontDeskNG object at ...>
  >>> queryAdapter(jack, IDesk, 'ng') #doctest: +ELLIPSIS
  <FrontDeskNG object at ...>

As you can see, the first argument should be adaptee then, the
interface which should be provided by component and last the name of
adapter component.

If you try to lookup the component with an name not used for
registration but for same adaptee and interface, the lookup will fail.
Here is how the two methods works in such a case::

  >>> getAdapter(jack, IDesk, 'not-exists') #doctest: +ELLIPSIS
  Traceback (most recent call last):
  ...
  ComponentLookupError: ...
  >>> reg = queryAdapter(jack,
  ...           IDesk, 'not-exists') #doctest: +ELLIPSIS
  >>> reg is None
  True

As you can see above, ``getAdapter`` raised a ``ComponentLookupError``
exception, but ``queryAdapter`` returned `None` when lookup failed.

The third argument, the name of registration, is optional.  If the
third argument is not given it will default to empty string ('').
Since there is no component registered with an empty string,
``getAdapter`` will raise ``ComponentLookupError`` .  Similarly
``queryAdapter`` will return `None`, see yourself how it works::

  >>> getAdapter(jack, IDesk) #doctest: +ELLIPSIS
  Traceback (most recent call last):
  ...
  ComponentLookupError: ...
  >>> reg = queryAdapter(jack, IDesk) #doctest: +ELLIPSIS
  >>> reg is None
  True

In this section you have learned how to register a simple adapter and
how to retrieve it from component registry.  These kind of adapters is
called single adapter, because it adapts only one adaptee.  If an
adapter adapts more that one adaptee, then it is called multi
adapter.


Retrieving adapter using interface
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Adapters can be directly retrieved using interfaces, but it will only
work for non-named single adapters.  The first argument is the adaptee
and the second argument is a keyword argument.  If adapter lookup
fails, second argument will be returned.

  >>> IDesk(jack, alternate='default-output')
  'default-output'

  Keyword name can be omitted:

  >>> IDesk(jack, 'default-output')
  'default-output'

  If second argument is not given, it will raise `TypeError`:

  >>> IDesk(jack) #doctest: +NORMALIZE_WHITESPACE +ELLIPSIS
  Traceback (most recent call last):
  ...
  TypeError: ('Could not adapt',
    <Guest object at ...>,
    <InterfaceClass __builtin__.IDesk>)

  Here `FrontDeskNG` is registered without name:

  >>> gsm.registerAdapter(FrontDeskNG)

  Now the adapter lookup should succeed:

  >>> IDesk(jack, 'default-output') #doctest: +ELLIPSIS
  <FrontDeskNG object at ...>

For simple cases, you may use interface to get adapter components.


Adapter pattern
~~~~~~~~~~~~~~~

The adapter concept in Zope Component Architecture and the classic
`adapter pattern` as described in Design Patterns book are very
similar.  But the intent of ZCA adapter usage is more wider than the
`adapter pattern` itself.  The intent of `adapter pattern` is to
convert the interface of a class into another interface clients
expect.  This allows classes work together that couldn't otherwise
because of incompatible interfaces.  But in the `motivation` section
of Design Patterns book, GoF says: "Often the adapter is responsible
for functionality the adapted class doesn't provide".  ZCA adapter has
more focus on adding functionality than creating a new interface for
an adapted object (adaptee).  ZCA adapter lets adapter classes extend
functionality by adding methods.  (It would be interesting to note
that `Adapter` was known as `Feature` in earlier stage of ZCA
design. ) [#feature]_

.. [#feature] Thread discussing renaming of `Feature` to `Adapter`:
   http:https://mail.zope.org/pipermail/zope3-dev/2001-December/000008.html

The above paragraph has a quote from Gang of Four book, it ends like
this: " ...adapted class doesn't provide".  But in the next sentence I
used "adapted object" instead of "adapted class", because GoF
describes about two variants of adapters based on implementations.
The first one is called `class adapter` and the other one is called
`object adapter`.  A class adapter uses multiple inheritance to adapt
one interface to another, on the other hand an object adapter relies
on object composition.  ZCA adapter is following object adapter
pattern, which use delegation as a mechanism for composition.  GoF's
second principle of object-oriented design goes like this: "Favor
object composition over class inheritance".  For more details about
this subject please read Design Patterns book.

The major attraction of ZCA adapter are the explicit interface for
components and the component registry.  ZCA adapter components are
registered in component registry and looked up by client objects using
interface and name when required.


Utility
-------


Introduction
~~~~~~~~~~~~

Now you know the concept of interface, adapter and component registry.
Sometimes it would be useful to register an object which is not
adapting anything.  Database connection, XML parser, object returning
unique Ids etc. are examples of these kinds of objects.  These kind of
components provided by the ZCA are called ``utility`` components.

Utilities are just objects that provide an interface and that are
looked up by an interface and a name.  This approach creates a global
registry by which instances can be registered and accessed by
different parts of your application, with no need to pass the
instances around as parameters.

You need not to register all component instances like this.  Only
register components which you want to make replaceable.


Simple utility
~~~~~~~~~~~~~~

A utility can be registered with a name or without a name.  A utility
registered with a name is called named utility, which you will see in
the next section.  Before implementing the utility, as usual, define
its interface.  Here is a `greeter` interface::

  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IGreeter(Interface):
  ...
  ...     def greet(name):
  ...         """Say hello"""

Like an adapter a utility may have more than one implementation.  Here
is a possible implementation of the above interface::

  >>> class Greeter(object):
  ...
  ...     implements(IGreeter)
  ...
  ...     def greet(self, name):
  ...         return "Hello " + name

The actual utility will be an instance of this class.  To use this
utility, you have to register it, later you can query it using the ZCA
API.  You can register an instance of this class (`utility`) using
``registerUtility``::

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> greet = Greeter()
  >>> gsm.registerUtility(greet, IGreeter)

In this example you registered the utility as providing the `IGreeter`
interface.  You can look the interface up with either `queryUtility`
or `getUtility`::

  >>> from zope.component import queryUtility
  >>> from zope.component import getUtility

  >>> queryUtility(IGreeter).greet('Jack')
  'Hello Jack'

  >>> getUtility(IGreeter).greet('Jack')
  'Hello Jack'

As you can see, adapters are normally classes, but utilities are
normally instances of classes.  Only once you are creating the
instance of a utility class, but adapter instances are dynamically
created whenever you query for it.


Named utility
~~~~~~~~~~~~~

When registering a utility component, like adapter, you can use a
name.  As mentioned in the previous section, a utility registered with
a particular name is called named utility.

This is how you can register the `greeter` utility with a name::

  >>> greet = Greeter()
  >>> gsm.registerUtility(greet, IGreeter, 'new')

In this example you registered the utility with a name as providing
the `IGreeter` interface.  You can look up the interface with either
`queryUtility` or `getUtility`::

  >>> from zope.component import queryUtility
  >>> from zope.component import getUtility

  >>> queryUtility(IGreeter, 'new').greet('Jill')
  'Hello Jill'

  >>> getUtility(IGreeter, 'new').greet('Jill')
  'Hello Jill'

As you can see here, while querying you have to use the `name` as
second argument.

Calling `getUtility` function without a name (second argument) is
equivalent to calling with an empty string as the name.  Because, the
default value for second (keyword) argument is an empty string.  Then,
component lookup mechanism will try to find the component with name as
empty string, and it will fail.  When component lookup fails it will
raise ``ComponentLookupError`` exception.  Remember, it will not
return some random component registered with some other name.  The
adapter look up functions, `getAdapter` and `queryAdapter` also works
similarly.


Factory
~~~~~~~

A ``Factory`` is a utility component which provides ``IFactory``
interface.

To create a factory, first define the interface of the object::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IDatabase(Interface):
  ...
  ...     def getConnection():
  ...         """Return connection object"""

Here is fake implementation of `IDatabase` interface::

  >>> class FakeDb(object):
  ...
  ...     implements(IDatabase)
  ...
  ...     def getConnection(self):
  ...         return "connection"

You can create a factory using ``zope.component.factory.Factory``::

  >>> from zope.component.factory import Factory

  >>> factory = Factory(FakeDb, 'FakeDb')

Now you can register it like this::

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> from zope.component.interfaces import IFactory
  >>> gsm.registerUtility(factory, IFactory, 'fakedb')

To use the factory, you may do it like this::

  >>> from zope.component import queryUtility
  >>> queryUtility(IFactory, 'fakedb')() #doctest: +ELLIPSIS
  <FakeDb object at ...>

There is a shortcut to use factory::

  >>> from zope.component import createObject
  >>> createObject('fakedb') #doctest: +ELLIPSIS
  <FakeDb object at ...>


Advanced adapters
-----------------

This chapter discuss some advanced adapters like multi adapter,
subscription adapter and handler.


Multi adapter
~~~~~~~~~~~~~

A simple adapter normally adapts only one object, but an adapter may
adapt more than one object.  If an adapter adapts more than one
objects, it is called `multi-adapter`.

::

  >>> from zope.interface import Interface
  >>> from zope.interface import implements
  >>> from zope.component import adapts

  >>> class IAdapteeOne(Interface):
  ...     pass

  >>> class IAdapteeTwo(Interface):
  ...     pass

  >>> class IFunctionality(Interface):
  ...     pass

  >>> class MyFunctionality(object):
  ...     implements(IFunctionality)
  ...     adapts(IAdapteeOne, IAdapteeTwo)
  ...
  ...     def __init__(self, one, two):
  ...         self.one = one
  ...         self.two = two

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> gsm.registerAdapter(MyFunctionality)

  >>> class One(object):
  ...     implements(IAdapteeOne)

  >>> class Two(object):
  ...     implements(IAdapteeTwo)

  >>> one = One()
  >>> two = Two()

  >>> from zope.component import getMultiAdapter

  >>> getMultiAdapter((one,two), IFunctionality) #doctest: +ELLIPSIS
  <MyFunctionality object at ...>

  >>> myfunctionality = getMultiAdapter((one,two), IFunctionality)
  >>> myfunctionality.one #doctest: +ELLIPSIS
  <One object at ...>
  >>> myfunctionality.two #doctest: +ELLIPSIS
  <Two object at ...>


Subscription adapter
~~~~~~~~~~~~~~~~~~~~

Unlike regular adapters, subscription adapters are used when we want
all of the adapters that adapt an object to a particular interface.
Subscription adapter is also known as `subscriber`.

Consider a validation problem.  We have objects and we want to assess
whether they meet some sort of standards.  We define a validation
interface::

  >>> from zope.interface import Interface
  >>> from zope.interface import Attribute
  >>> from zope.interface import implements

  >>> class IValidate(Interface):
  ...
  ...     def validate(ob):
  ...         """Determine whether the object is valid
  ...
  ...         Return a string describing a validation problem.
  ...         An empty string is returned to indicate that the
  ...         object is valid.
  ...         """

Perhaps we have documents::

  >>> class IDocument(Interface):
  ...
  ...     summary = Attribute("Document summary")
  ...     body = Attribute("Document text")

  >>> class Document(object):
  ...
  ...     implements(IDocument)
  ...
  ...     def __init__(self, summary, body):
  ...         self.summary, self.body = summary, body

Now, we may want to specify various validation rules for
documents. For example, we might require that the summary be a single
line::

  >>> from zope.component import adapts

  >>> class SingleLineSummary:
  ...
  ...     adapts(IDocument)
  ...     implements(IValidate)
  ...
  ...     def __init__(self, doc):
  ...         self.doc = doc
  ...
  ...     def validate(self):
  ...         if '\n' in self.doc.summary:
  ...             return 'Summary should only have one line'
  ...         else:
  ...             return ''

Or we might require the body to be at least 1000 characters in length::

  >>> class AdequateLength(object):
  ...
  ...     adapts(IDocument)
  ...     implements(IValidate)
  ...
  ...     def __init__(self, doc):
  ...         self.doc = doc
  ...
  ...     def validate(self):
  ...         if len(self.doc.body) < 1000:
  ...             return 'too short'
  ...         else:
  ...             return ''

We can register these as subscription adapters::

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> gsm.registerSubscriptionAdapter(SingleLineSummary)
  >>> gsm.registerSubscriptionAdapter(AdequateLength)

We can then use the subscribers to validate objects::

  >>> from zope.component import subscribers

  >>> doc = Document("A\nDocument", "blah")
  >>> [adapter.validate()
  ...  for adapter in subscribers([doc], IValidate)
  ...  if adapter.validate()]
  ['Summary should only have one line', 'too short']

  >>> doc = Document("A\nDocument", "blah" * 1000)
  >>> [adapter.validate()
  ...  for adapter in subscribers([doc], IValidate)
  ...  if adapter.validate()]
  ['Summary should only have one line']

  >>> doc = Document("A Document", "blah")
  >>> [adapter.validate()
  ...  for adapter in subscribers([doc], IValidate)
  ...  if adapter.validate()]
  ['too short']


Handler
~~~~~~~

Handlers are subscription adapter factories that don't produce
anything.  They do all of their work when called.  Handlers are
typically used to handle events.  Handlers are also known as event
subscribers or event subscription adapters.

Event subscribers are different from other subscription adapters in
that the caller of event subscribers doesn't expect to interact with
them in any direct way.  For example, an event publisher doesn't
expect to get any return value.  Because subscribers don't need to
provide an API to their callers, it is more natural to define them
with functions, rather than classes.  For example, in a
document-management system, we might want to record creation times for
documents::

  >>> import datetime

  >>> def documentCreated(event):
  ...     event.doc.created = datetime.datetime.utcnow()

In this example, we have a function that takes an event and performs
some processing.  It doesn't actually return anything.  This is a
special case of a subscription adapter that adapts an event to
nothing.  All of the work is done when the adapter "factory" is
called.  We call subscribers that don't actually create anything
"handlers".  There are special APIs for registering and calling them.

To register the subscriber above, we define a document-created event::

  >>> from zope.interface import Interface
  >>> from zope.interface import Attribute
  >>> from zope.interface import implements

  >>> class IDocumentCreated(Interface):
  ...
  ...     doc = Attribute("The document that was created")

  >>> class DocumentCreated(object):
  ...
  ...     implements(IDocumentCreated)
  ...
  ...     def __init__(self, doc):
  ...         self.doc = doc

We'll also change our handler definition to::

  >>> def documentCreated(event):
  ...     event.doc.created = datetime.datetime.utcnow()

  >>> from zope.component import adapter

  >>> @adapter(IDocumentCreated)
  ... def documentCreated(event):
  ...     event.doc.created = datetime.datetime.utcnow()

This marks the handler as an adapter of `IDocumentCreated` events.

Now we'll register the handler::

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> gsm.registerHandler(documentCreated)

Now, we can create an event and use the `handle` function to call
handlers registered for the event::

  >>> from zope.component import handle

  >>> handle(DocumentCreated(doc))
  >>> doc.created.__class__.__name__
  'datetime'


ZCA usage in Zope
-----------------

Zope Component Architecture is used in both Zope 3 and Zope 2.  This
chapter will go through usage of the ZCA in Zope.


ZCML
~~~~

The **Zope Configuration Markup Language (ZCML)** is an XML based
configuration system for registration of components.  So, instead of
using Python API for registration, you can use ZCML.  But to use
ZCML, you will be required to install more dependency packages.

You can install ``zope.component`` with ZCML support using
easy_install like this::

  $ easy_install "zope.component [zcml]"

A ZCML file should start with ``configure`` directive with
appropriate namespace declaration::

  <configure xmlns="http:https://namespaces.zope.org/zope">
  ...
  </configure>

The `adapter` directive can be used to register adapters::

  <adapter
      factory=".company.EmployeeSalary"
      provides=".interfaces.ISalary"
      for=".interfaces.IEmployee"
      />

The `provides` and `for` attributes are optional, provided you have
declared it in the implementation::

  <adapter
      factory=".company.EmployeeSalary"
      />

If you want to register the component as a named adapter, you can use
the `name` attribute::

  <adapter
      factory=".company.EmployeeSalary"
      name="salary"
      />

A utility can be registered using `utility` directive.  To register
an utility::

  <utility
      component=".database.connection"
      provides=".interfaces.IConnection"
      />

The `provides` attribute is optional, provided you have declared it in
the implementation::

  <configure xmlns="http:https://namespaces.zope.org/zope">

  <utility
      component=".database.connection"
      />

If you want to register the component as a named utility, you can use
the `name` attribute::

  <utility
      component=".database.connection"
      name="db_connection"
      />

Instead of directly using the component, you can also give a factory::

  <utility
      factory=".database.Connection"
      />


Overrides
~~~~~~~~~

When you register components using Python API (``register*`` methods),
the last registered component will replace previously registered
component, if both are registered with same type of arguments.  For
example, consider this example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface

  >>> class IA(Interface):
  ...     pass

  >>> class IP(Interface):
  ...     pass

  >>> from zope.interface import implements
  >>> from zope.component import adapts

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> class AP(object):
  ...
  ...     implements(IP)
  ...     adapts(IA)
  ...
  ...     def __init__(self, context):
  ...         self.context = context

  >>> class AP2(object):
  ...
  ...     implements(IP)
  ...     adapts(IA)
  ...
  ...     def __init__(self, context):
  ...         self.context = context

  >>> class A(object):
  ...
  ...     implements(IA)

  >>> a = A()
  >>> ap = AP(a)

  >>> gsm.registerAdapter(AP)

  >>> getAdapter(a, IP) #doctest: +ELLIPSIS
  <AP object at ...>

If you register another adapter, the existing one will be replaced::

  >>> gsm.registerAdapter(AP2)

  >>> getAdapter(a, IP) #doctest: +ELLIPSIS
  <AP2 object at ...>

But when registering components using ZCML, the second registration
will raise a conflict error.  This is a hint for you, otherwise there
is a chance for overriding registration by mistake.  This may lead to
hard to track bugs in your system.  So, using ZCML is a win for the
application.

Sometimes you will be required to override existing registration.
ZCML provides ``includeOverrides`` directive for this.  Using this,
you can write your overrides in a separate file::

  <includeOverrides file="overrides.zcml" />


NameChooser
~~~~~~~~~~~

Location: `zope.app.container.contained.NameChooser`

This is an adapter for choosing a unique name for an object inside a
container.

The registration of adapter is like this::

  <adapter
      provides=".interfaces.INameChooser"
      for="zope.app.container.interfaces.IWriteContainer"
      factory=".contained.NameChooser"
      />

From the registration, you can see that the adaptee is a
``IWriteContainer`` and the adapter provides ``INameChooser``.

This adapter provides a very convenient functionality for Zope
programmers.  The main implementations of ``IWriteContainer`` in
Zope 3 are ``zope.app.container.BTreeContainer`` and
``zope.app.folder.Folder``.  Normally you will be inheriting from
these implementations for creating your own container classes.
Suppose there is no interface called ``INameChooser`` and
adapter, then you will be required to implement this functionality
for every implementations separately.


LocationPhysicallyLocatable
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Location:
``zope.location.traversing.LocationPhysicallyLocatable``

This adapter is frequently used in Zope 3 applications, but
normally it is called through an API in ``zope.traversing.api``.
(Some old code even use ``zope.app.zapi`` functions, which is
again one more indirection)

The registration of adapter is like this::

  <adapter
      factory="zope.location.traversing.LocationPhysicallyLocatable"
      />

The interface provided and adaptee interface is given in the
implementation.

Here is the beginning of implementation::

  class LocationPhysicallyLocatable(object):
      """Provide location information for location objects
      """
      zope.component.adapts(ILocation)
      zope.interface.implements(IPhysicallyLocatable)
      ...

Normally, almost all persistent objects in Zope 3 application
will be providing the ``ILocation`` interface.  This interface
has only two attribute, ``__parent__`` and ``__name__``.  The
``__parent__`` is the parent in the location hierarchy.  And
``__name__`` is the name within the parent.

The ``IPhysicallyLocatable`` interface has four methods:
``getRoot``, ``getPath``, ``getName``, and ``getNearestSite``.

  - ``getRoot`` function will return the physical root object.

  - ``getPath`` return the physical path to the object as a
    string.

  - ``getName`` return the last segment of the physical path.

  - ``getNearestSite`` return the site the object is contained
    in.  If the object is a site, the object itself is returned.

If you learn Zope 3, you can see that these are the important
things which you required very often.  To understand the beauty
of this system, you must see how Zope 2 actually get the physical
root object and how it is implemented.  There is a method called
``getPhysicalRoot`` virtually for all container objects.


DefaultSized
~~~~~~~~~~~~

Location: ``zope.size.DefaultSized``

This adapter is just a default implementation of ``ISized`` interface.
This adapter is registered for all kind of objects.  If you want to
register this adapter for a particular interface, then you have to
override this registration for your implementation.

The registration of adapter is like this::

  <adapter
      for="*"
      factory="zope.size.DefaultSized"
      provides="zope.size.interfaces.ISized"
      permission="zope.View"
      />

As you can see, the adaptee interface is `*`, so it can adapt any kind
of objects.

The ``ISized`` is a simple interface with two method contracts::

  class ISized(Interface):

      def sizeForSorting():
          """Returns a tuple (basic_unit, amount)

          Used for sorting among different kinds of sized objects.
          'amount' need only be sortable among things that share the
          same basic unit."""

      def sizeForDisplay():
          """Returns a string giving the size.
          """

You can see another ``ISized`` adapter registered for ``IZPTPage`` in
``zope.app.zptpage`` package.


ZopeVersionUtility
~~~~~~~~~~~~~~~~~~

Location: ``zope.app.applicationcontrol.ZopeVersionUtility``

This utility gives version of the running Zope.

The registration goes like this::

  <utility
      component=".zopeversion.ZopeVersionUtility"
      provides=".interfaces.IZopeVersion" />

The interface provided, ``IZopeVersion``, has only one method named
``getZopeVersion``.  This method return a string containing the Zope
version (possibly including SVN information).

The default implementation, ``ZopeVersionUtility``, get version info
from a file ``version.txt`` in `zope/app` directory.  If Zope is
running from subversion checkout, it will show the latest revision
number.  If none of the above works it will set it to:
`Development/Unknown`.


Case study
----------

.. note::

  This chapter is not yet completed.  Please send your suggestions !

Introduction
~~~~~~~~~~~~

This chapter demonstrates creating a desktop application using PyGTK
GUI library and the ZCA.  This application also use two different
kinds of data persistence mechanisms, one object database (ZODB) & and
another relational database (SQLite).  However, practically, only one
storage can be used for a particular installation.  The reason for
using two different persistence mechanisms is to demonstrate how to
use the ZCA to glue components.  Majority of the code in this
application is related to PyGTK.

As the application grows you may use the ZCA components wherever you
want pluggability or extensibility.  Use plain Python objects directly
where you do not required pluggability or extensibility.

There is no difference in using ZCA for web or desktop or any other
kind of application or framework.  It is better to follow a
convention for the location from where you are going to register
components.  This application use a convention, which can be extended
by putting registration of similar components in separate modules and
later import them from main registration module.  In this application
the main component registration module is `register.py`.

Source code of this application can be downloaded from:
http:https://www.muthukadan.net/downloads/zcalib.tar.bz2


Use cases
~~~~~~~~~

The application we are going to discuss here is a library management
system with minimal features.  The requirements can be summarized like
this:

- Add members with a unique number and name.

- Add books with barcode, author & title

- Issue books

- Return books


The application can be designed in such a way that major features can
be accessed from a single window.  The main window for accessing all
these features can be designed like this:

.. image:: mainwindow.png
   :align: center

From member window, user should be able to manage members.  So, it
should be possible to *add*, *update* and *delete* members as shown
in the below picture:

.. image:: memberwindow.png
   :align: center

Similar to member window, the catalog window allows user to *add*,
*edit* and *delete* books:

.. image:: catalogwindow.png
   :align: center

The circulation window should have the facility for issuing and
returning books:

.. image:: circulationwindow.png
   :align: center


Overview of PyGTK code
~~~~~~~~~~~~~~~~~~~~~~

As you can see in the code, most of the code are related to PyGTK.
The code structure is very similar for different windows.  The windows
of this application are designed using Glade GUI builder.  You should
give proper names for widgets you are going to use from code.  In the
main window, all menu entries has names like: circulation, catalog,
member, quit & about.

The ``gtk.glade.XML`` class is used to parse glade file, this will
create GUI widget objects.  This is how to parse and access objects::

  import gtk.glade
  xmlobj = gtk.glade.XML('/path/to/file.glade')
  widget = xmlobj.get_widget('widget_name')

In the mainwindow.py, you can see code like this::

  curdir = os.path.abspath(os.path.dirname(__file__))
  xml = os.path.join(curdir, 'glade', 'mainwindow.glade')
  xmlobj = gtk.glade.XML(xml)

  self.mainwindow = xmlobj.get_widget('mainwindow')

The name of main window widget is `mainwindow`.  Similarly, other
widgets are retrieved like this::

  circulation = xmlobj.get_widget('circulation')
  member = xmlobj.get_widget('member')
  quit = xmlobj.get_widget('quit')
  catalog = xmlobj.get_widget('catalog')
  about = xmlobj.get_widget('about')

Then, these widgets are connected for some events::

  self.mainwindow.connect('delete_event', self.delete_event)
  quit.connect('activate', self.delete_event)
  circulation.connect('activate', self.on_circulation_activate)
  member.connect('activate', self.on_member_activate)
  catalog.connect('activate', self.on_catalog_activate)
  about.connect('activate', self.on_about_activate)

The `delete_event` is the event when the window is closing using the
window close button.  The `activate` event is emitted when the menu is
selected.  The widgets are connected to some callback functions for
some events.

You can see from the above code that, main window is connected to
`on_delete_event` method for `delete_event`.  The `quit` widget is
also connected to same method for `activate` event::

    def on_delete_event(self, *args):
        gtk.main_quit()

The callback function just call `main_quit` function


The code
~~~~~~~~

This is the `zcalib.py`::

  import registry
  import mainwindow

  if __name__ == '__main__':
      registry.initialize()
      try:
          mainwindow.main()
      except KeyboardInterrupt:
          import sys
          sys.exit(1)

Here, two modules are imported `registry` and `mainwindow`.  Then,
registry is initialized and mainwindow's `main` function is called.
If user is trying to exit application using `Ctrl+C`, system will exit
normally, that's why we catched `KeyboardInterrupt` exception.

This is the `registry.py`::

  import sys
  from zope.component import getGlobalSiteManager

  from interfaces import IMember
  from interfaces import IBook
  from interfaces import ICirculation
  from interfaces import IDbOperation


  def initialize_rdb():
      from interfaces import IRelationalDatabase
      from relationaldatabase import RelationalDatabase
      from member import MemberRDbOperation
      from catalog import BookRDbOperation
      from circulation import CirculationRDbOperation

      gsm = getGlobalSiteManager()
      db = RelationalDatabase()
      gsm.registerUtility(db, IRelationalDatabase)

      gsm.registerAdapter(MemberRDbOperation,
                          (IMember,),
                          IDbOperation)

      gsm.registerAdapter(BookRDbOperation,
                          (IBook,),
                          IDbOperation)

      gsm.registerAdapter(CirculationRDbOperation,
                          (ICirculation,),
                          IDbOperation)

  def initialize_odb():
      from interfaces import IObjectDatabase
      from objectdatabase import ObjectDatabase
      from member import MemberODbOperation
      from catalog import BookODbOperation
      from circulation import CirculationODbOperation

      gsm = getGlobalSiteManager()
      db = ObjectDatabase()
      gsm.registerUtility(db, IObjectDatabase)

      gsm.registerAdapter(MemberODbOperation,
                          (IMember,),
                          IDbOperation)

      gsm.registerAdapter(BookODbOperation,
                          (IBook,),
                          IDbOperation)

      gsm.registerAdapter(CirculationODbOperation,
                          (ICirculation,),
                          IDbOperation)

  def check_use_relational_db():
      use_rdb = False
      try:
          arg = sys.argv[1]
          if arg == '-r':
              return True
      except IndexError:
          pass
      return use_rdb

  def initialize():
      use_rdb = check_use_relational_db()
      if use_rdb:
          initialize_rdb()
      else:
          initialize_odb()

Look at the `initialize` function which we are calling from the main
module, `zcalib.py`.  The `initialize` function first check which db
to use, relational database (RDB) or object database (ODB) and this
checking is done at `check_use_relational_db` function.  If `-r`
option is given at command line, it will call `initialize_rdb`
otherwise, `initialize_odb`.  If the RDB function is called, it will
setup all components related to RDB.  On the other hand, if the ODB
function is called, it will setup all components related to ODB.

Here is the `mainwindow.py`::

  import os
  import gtk
  import gtk.glade

  from circulationwindow import circulationwindow
  from catalogwindow import catalogwindow
  from memberwindow import memberwindow

  class MainWindow(object):

      def __init__(self):
          curdir = os.path.abspath(os.path.dirname(__file__))
          xml = os.path.join(curdir, 'glade', 'mainwindow.glade')
          xmlobj = gtk.glade.XML(xml)

          self.mainwindow = xmlobj.get_widget('mainwindow')
          circulation = xmlobj.get_widget('circulation')
          member = xmlobj.get_widget('member')
          quit = xmlobj.get_widget('quit')
          catalog = xmlobj.get_widget('catalog')
          about = xmlobj.get_widget('about')

          self.mainwindow.connect('delete_event', self.delete_event)
          quit.connect('activate', self.delete_event)

          circulation.connect('activate', self.on_circulation_activate)
          member.connect('activate', self.on_member_activate)
          catalog.connect('activate', self.on_catalog_activate)
          about.connect('activate', self.on_about_activate)

      def delete_event(self, *args):
          gtk.main_quit()

      def on_circulation_activate(self, *args):
          circulationwindow.show_all()

      def on_member_activate(self, *args):
          memberwindow.show_all()

      def on_catalog_activate(self, *args):
          catalogwindow.show_all()

      def on_about_activate(self, *args):
          pass

      def run(self):
          self.mainwindow.show_all()

  def main():
      mainwindow = MainWindow()
      mainwindow.run()
      gtk.main()


The `main` function here creates an instance of `MainWindow` class,
which will initialize all widgets.

Here is the `memberwindow.py`::

  import os
  import gtk
  import gtk.glade

  from zope.component import getAdapter

  from components import Member
  from interfaces import IDbOperation


  class MemberWindow(object):

      def __init__(self):
          curdir = os.path.abspath(os.path.dirname(__file__))
          xml = os.path.join(curdir, 'glade', 'memberwindow.glade')
          xmlobj = gtk.glade.XML(xml)

          self.memberwindow = xmlobj.get_widget('memberwindow')
          self.number = xmlobj.get_widget('number')
          self.name = xmlobj.get_widget('name')
          add = xmlobj.get_widget('add')
          update = xmlobj.get_widget('update')
          delete = xmlobj.get_widget('delete')
          close = xmlobj.get_widget('close')
          self.treeview = xmlobj.get_widget('treeview')

          self.memberwindow.connect('delete_event', self.on_delete_event)
          add.connect('clicked', self.on_add_clicked)
          update.connect('clicked', self.on_update_clicked)
          delete.connect('clicked', self.on_delete_clicked)
          close.connect('clicked', self.on_delete_event)

          self.initialize_list()

      def show_all(self):
          self.populate_list_store()
          self.memberwindow.show_all()

      def populate_list_store(self):
          self.list_store.clear()
          member = Member()
          memberdboperation = getAdapter(member, IDbOperation)
          members = memberdboperation.get()
          for member in members:
              number = member.number
              name = member.name
              self.list_store.append((member, number, name,))

      def on_delete_event(self, *args):
          self.memberwindow.hide()
          return True

      def initialize_list(self):
          self.list_store = gtk.ListStore(object, str, str)
          self.treeview.set_model(self.list_store)
          tvcolumn = gtk.TreeViewColumn('Member Number')
          self.treeview.append_column(tvcolumn)

          cell = gtk.CellRendererText()
          tvcolumn.pack_start(cell, True)
          tvcolumn.add_attribute(cell, 'text', 1)

          tvcolumn = gtk.TreeViewColumn('Member Name')
          self.treeview.append_column(tvcolumn)

          cell = gtk.CellRendererText()
          tvcolumn.pack_start(cell, True)
          tvcolumn.add_attribute(cell, 'text', 2)

      def on_add_clicked(self, *args):
          number = self.number.get_text()
          name = self.name.get_text()
          member = Member()
          member.number = number
          member.name = name
          self.add(member)
          self.list_store.append((member, number, name,))

      def add(self, member):
          memberdboperation = getAdapter(member, IDbOperation)
          memberdboperation.add()

      def on_update_clicked(self, *args):
          number = self.number.get_text()
          name = self.name.get_text()
          treeselection = self.treeview.get_selection()
          model, iter = treeselection.get_selected()
          if not iter:
              return
          member = self.list_store.get_value(iter, 0)
          member.number = number
          member.name = name
          self.update(member)
          self.list_store.set(iter, 1, number, 2, name)

      def update(self, member):
          memberdboperation = getAdapter(member, IDbOperation)
          memberdboperation.update()

      def on_delete_clicked(self, *args):
          treeselection = self.treeview.get_selection()
          model, iter = treeselection.get_selected()
          if not iter:
              return
          member = self.list_store.get_value(iter, 0)
          self.delete(member)
          self.list_store.remove(iter)

      def delete(self, member):
          memberdboperation = getAdapter(member, IDbOperation)
          memberdboperation.delete()

  memberwindow = MemberWindow()

Here is the `components.py`::

  from zope.interface import implements

  from interfaces import IBook
  from interfaces import IMember
  from interfaces import ICirculation

  class Book(object):

      implements(IBook)

      barcode = ""
      title = ""
      author = ""

  class Member(object):

      implements(IMember)

      number = ""
      name = ""

  class Circulation(object):

      implements(ICirculation)

      book = Book()
      member = Member()

Here is the `interfaces.py`::

  from zope.interface import Interface
  from zope.interface import Attribute


  class IBook(Interface):

      barcode = Attribute("Barcode")
      author = Attribute("Author of book")
      title = Attribute("Title of book")


  class IMember(Interface):

      number = Attribute("ID number")
      name = Attribute("Name of member")


  class ICirculation(Interface):

      book = Attribute("A book")
      member = Attribute("A member")


  class IRelationalDatabase(Interface):

      def commit():
          pass

      def rollback():
          pass

      def cursor():
          pass

      def get_next_id():
          pass


  class IObjectDatabase(Interface):

      def commit():
          pass

      def rollback():
          pass

      def container():
          pass

      def get_next_id():
          pass


  class IDbOperation(Interface):

      def get():
          pass

      def add():
          pass

      def update():
          pass

      def delete():
          pass

Here is the `member.py`::

  from zope.interface import implements
  from zope.component import getUtility
  from zope.component import adapts

  from components import Member

  from interfaces import IRelationalDatabase
  from interfaces import IObjectDatabase
  from interfaces import IMember
  from interfaces import IDbOperation


  class MemberRDbOperation(object):

      implements(IDbOperation)
      adapts(IMember)

      def __init__(self, member):
          self.member = member

      def get(self):
          db = getUtility(IRelationalDatabase)
          cr = db.cursor()
          number = self.member.number
          if number:
              cr.execute("""SELECT
                              id,
                              number,
                              name
                            FROM members
                            WHERE number = ?""",
                         (number,))
          else:
              cr.execute("""SELECT
                              id,
                              number,
                              name
                            FROM members""")
          rst = cr.fetchall()
          cr.close()
          members = []
          for record in rst:
              id = record['id']
              number = record['number']
              name = record['name']
              member = Member()
              member.id = id
              member.number = number
              member.name = name
              members.append(member)
          return members

      def add(self):
          db = getUtility(IRelationalDatabase)
          cr = db.cursor()
          next_id = db.get_next_id("members")
          number = self.member.number
          name = self.member.name
          cr.execute("""INSERT INTO members
                          (id, number, name)
                        VALUES (?, ?, ?)""",
                     (next_id, number, name))
          cr.close()
          db.commit()
          self.member.id = next_id

      def update(self):
          db = getUtility(IRelationalDatabase)
          cr = db.cursor()
          number = self.member.number
          name = self.member.name
          id = self.member.id
          cr.execute("""UPDATE members
                          SET
                             number = ?,
                             name = ?
                        WHERE id = ?""",
                     (number, name, id))
          cr.close()
          db.commit()

      def delete(self):
          db = getUtility(IRelationalDatabase)
          cr = db.cursor()
          id = self.member.id
          cr.execute("""DELETE FROM members
                        WHERE id = ?""",
                     (id,))
          cr.close()
          db.commit()


  class MemberODbOperation(object):

      implements(IDbOperation)
      adapts(IMember)

      def __init__(self, member):
          self.member = member

      def get(self):
          db = getUtility(IObjectDatabase)
          zcalibdb = db.container()
          members = zcalibdb['members']
          return members.values()

      def add(self):
          db = getUtility(IObjectDatabase)
          zcalibdb = db.container()
          members = zcalibdb['members']
          number = self.member.number
          if number in [x.number for x in members.values()]:
              db.rollback()
              raise Exception("Duplicate key")
          next_id = db.get_next_id('members')
          self.member.id = next_id
          members[next_id] = self.member
          db.commit()

      def update(self):
          db = getUtility(IObjectDatabase)
          zcalibdb = db.container()
          members = zcalibdb['members']
          id = self.member.id
          members[id] = self.member
          db.commit()

      def delete(self):
          db = getUtility(IObjectDatabase)
          zcalibdb = db.container()
          members = zcalibdb['members']
          id = self.member.id
          del members[id]
          db.commit()


PySQLite
~~~~~~~~

ZODB
~~~~

Conclusions
~~~~~~~~~~~

Reference
---------


adaptedBy
~~~~~~~~~

This function helps to find the adapted interfaces.

 - Location: ``zope.component``

 - Signature: `adaptedBy(object)`

Example::

  >>> from zope.interface import implements
  >>> from zope.component import adapts
  >>> from zope.component import adaptedBy

  >>> class FrontDeskNG(object):
  ...
  ...     implements(IDesk)
  ...     adapts(IGuest)
  ...
  ...     def __init__(self, guest):
  ...         self.guest = guest

  >>> adaptedBy(FrontDeskNG)
  (<InterfaceClass __builtin__.IGuest>,)


adapter
~~~~~~~

Adapters can be any callable object, you can use the `adapter`
decorator to declare that a callable object adapts some interfaces (or
classes)

 - Location: ``zope.component``

 - Signature: `adapter(*interfaces)`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface
  >>> from zope.interface import implementer
  >>> from zope.component import adapter
  >>> from zope.interface import implements

  >>> class IJob(Interface):
  ...     """A job"""

  >>> class Job(object):
  ...     implements(IJob)

  >>> class IPerson(Interface):
  ...
  ...     name = Attribute("Name")
  ...     job = Attribute("Job")

  >>> class Person(object):
  ...     implements(IPerson)
  ...
  ...     name = None
  ...     job = None

  >>> @implementer(IJob)
  ... @adapter(IPerson)
  ... def personJob(person):
  ...     return person.job

  >>> jack = Person()
  >>> jack.name = "Jack"
  >>> jack.job = Job()
  >>> personJob(jack) #doctest: +ELLIPSIS
  <Job object at ...>


adapts
~~~~~~

This function helps to declare adapter classes.

 - Location: ``zope.component``

 - Signature: `adapts(*interfaces)`

Example::

  >>> from zope.interface import implements
  >>> from zope.component import adapts

  >>> class FrontDeskNG(object):
  ...
  ...     implements(IDesk)
  ...     adapts(IGuest)
  ...
  ...     def __init__(self, guest):
  ...         self.guest = guest
  ...
  ...     def register(self):
  ...         next_id = get_next_id()
  ...         bookings_db[next_id] = {
  ...         'name': guest.name,
  ...         'place': guest.place,
  ...         'phone': guest.phone
  ...         }


alsoProvides
~~~~~~~~~~~~

Declare interfaces declared directly for an object.  The arguments
after the object are one or more interfaces.  The interfaces given are
added to the interfaces previously declared for the object.

 - Location: ``zope.interface``

 - Signature: `alsoProvides(object, *interfaces)`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface
  >>> from zope.interface import implements
  >>> from zope.interface import alsoProvides

  >>> class IPerson(Interface):
  ...
  ...     name = Attribute("Name of person")

  >>> class IStudent(Interface):
  ...
  ...     college = Attribute("Name of college")

  >>> class Person(object):
  ...
  ...     implements(IDesk)
  ...     name = u""

  >>> jack = Person()
  >>> jack.name = "Jack"
  >>> jack.college = "New College"
  >>> alsoProvides(jack, IStudent)

  You can test it like this:

  >>> from zope.interface import providedBy
  >>> IStudent in providedBy(jack)
  True


Attribute
~~~~~~~~~

Using this class, you can define normal attributes in an interface.

 - Location: ``zope.interface``

 - Signature: `Attribute(name, doc='')`

 - See also: `Interface`_

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface

  >>> class IPerson(Interface):
  ...
  ...     name = Attribute("Name of person")
  ...     email = Attribute("Email Address")


classImplements
~~~~~~~~~~~~~~~

Declare additional interfaces implemented for instances of a class.
The arguments after the class are one or more interfaces.  The
interfaces given are added to any interfaces previously declared.

 - Location: ``zope.interface``

 - Signature: `classImplements(cls, *interfaces)`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface
  >>> from zope.interface import implements
  >>> from zope.interface import classImplements

  >>> class IPerson(Interface):
  ...
  ...     name = Attribute("Name of person")

  >>> class IStudent(Interface):
  ...
  ...     college = Attribute("Name of college")

  >>> class Person(object):
  ...
  ...     implements(IDesk)
  ...     name = u""
  ...     college = u""

  >>> classImplements(Person, IStudent)
  >>> jack = Person()
  >>> jack.name = "Jack"
  >>> jack.college = "New College"

  You can test it like this:

  >>> from zope.interface import providedBy
  >>> IStudent in providedBy(jack)
  True


classImplementsOnly
~~~~~~~~~~~~~~~~~~~

Declare the only interfaces implemented by instances of a class.  The
arguments after the class are one or more interfaces.  The interfaces
given replace any previous declarations.

 - Location: ``zope.interface``

 - Signature: `classImplementsOnly(cls, *interfaces)`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface
  >>> from zope.interface import implements
  >>> from zope.interface import classImplementsOnly

  >>> class IPerson(Interface):
  ...
  ...     name = Attribute("Name of person")

  >>> class IStudent(Interface):
  ...
  ...     college = Attribute("Name of college")

  >>> class Person(object):
  ...
  ...     implements(IPerson)
  ...     college = u""

  >>> classImplementsOnly(Person, IStudent)
  >>> jack = Person()
  >>> jack.college = "New College"

  You can test it like this:

  >>> from zope.interface import providedBy
  >>> IPerson in providedBy(jack)
  False
  >>> IStudent in providedBy(jack)
  True


classProvides
~~~~~~~~~~~~~

Normally if a class implements a particular interface, the instance of
that class will provide the interface implemented by that class.  But
if you want a class to be provided by an interface, you can declare it
using ``classProvides`` function.

 - Location: ``zope.interface``

 - Signature: `classProvides(*interfaces)`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface
  >>> from zope.interface import classProvides

  >>> class IPerson(Interface):
  ...
  ...     name = Attribute("Name of person")

  >>> class Person(object):
  ...
  ...     classProvides(IPerson)
  ...     name = u"Jack"

  You can test it like this:

  >>> from zope.interface import providedBy
  >>> IPerson in providedBy(Person)
  True


ComponentLookupError
~~~~~~~~~~~~~~~~~~~~

This is the exception raised when a component lookup fails.

Example::

  >>> class IPerson(Interface):
  ...
  ...     name = Attribute("Name of person")

  >>> person = object()
  >>> getAdapter(person, IPerson, 'not-exists') #doctest: +ELLIPSIS
  Traceback (most recent call last):
  ...
  ComponentLookupError: ...


createObject
~~~~~~~~~~~~

Create an object using a factory.

Finds the named factory in the current site and calls it with the
given arguments.  If a matching factory cannot be found raises
``ComponentLookupError``.  Returns the created object.

A context keyword argument can be provided to cause the factory to be
looked up in a location other than the current site.  (Of course, this
means that it is impossible to pass a keyword argument named "context"
to the factory.

 - Location: ``zope.component``

 - Signature: `createObject(factory_name, *args, **kwargs)`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IDatabase(Interface):
  ...
  ...     def getConnection():
  ...         """Return connection object"""

  >>> class FakeDb(object):
  ...
  ...     implements(IDatabase)
  ...
  ...     def getConnection(self):
  ...         return "connection"

  >>> from zope.component.factory import Factory

  >>> factory = Factory(FakeDb, 'FakeDb')

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> from zope.component.interfaces import IFactory
  >>> gsm.registerUtility(factory, IFactory, 'fakedb')

  >>> from zope.component import createObject
  >>> createObject('fakedb') #doctest: +ELLIPSIS
  <FakeDb object at ...>


Declaration
~~~~~~~~~~~

Need not to use directly.


directlyProvidedBy
~~~~~~~~~~~~~~~~~~

This function will return the interfaces directly provided by the
given object.

 - Location: ``zope.interface``

 - Signature: `directlyProvidedBy(object)`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface

  >>> class IPerson(Interface):
  ...
  ...     name = Attribute("Name of person")

  >>> class IStudent(Interface):
  ...
  ...     college = Attribute("Name of college")

  >>> class ISmartPerson(Interface):
  ...     pass

  >>> class Person(object):
  ...
  ...     implements(IPerson)
  ...     name = u""

  >>> jack = Person()
  >>> jack.name = u"Jack"
  >>> jack.college = "New College"
  >>> alsoProvides(jack, ISmartPerson, IStudent)

  >>> from zope.interface import directlyProvidedBy

  >>> jack_dp = directlyProvidedBy(jack)
  >>> IPerson in jack_dp.interfaces()
  False
  >>> IStudent in jack_dp.interfaces()
  True
  >>> ISmartPerson in jack_dp.interfaces()
  True


directlyProvides
~~~~~~~~~~~~~~~~

Declare interfaces declared directly for an object.  The arguments
after the object are one or more interfaces.  The interfaces given
replace interfaces previously declared for the object.

 - Location: ``zope.interface``

 - Signature: `directlyProvides(object, *interfaces)`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface

  >>> class IPerson(Interface):
  ...
  ...     name = Attribute("Name of person")

  >>> class IStudent(Interface):
  ...
  ...     college = Attribute("Name of college")

  >>> class ISmartPerson(Interface):
  ...     pass

  >>> class Person(object):
  ...
  ...     implements(IPerson)
  ...     name = u""

  >>> jack = Person()
  >>> jack.name = u"Jack"
  >>> jack.college = "New College"
  >>> alsoProvides(jack, ISmartPerson, IStudent)

  >>> from zope.interface import directlyProvidedBy

  >>> jack_dp = directlyProvidedBy(jack)
  >>> ISmartPerson in jack_dp.interfaces()
  True
  >>> IPerson in jack_dp.interfaces()
  False
  >>> IStudent in jack_dp.interfaces()
  True
  >>> from zope.interface import providedBy

  >>> ISmartPerson in providedBy(jack)
  True

  >>> from zope.interface import directlyProvides
  >>> directlyProvides(jack, IStudent)

  >>> jack_dp = directlyProvidedBy(jack)
  >>> ISmartPerson in jack_dp.interfaces()
  False
  >>> IPerson in jack_dp.interfaces()
  False
  >>> IStudent in jack_dp.interfaces()
  True

  >>> ISmartPerson in providedBy(jack)
  False


getAdapter
~~~~~~~~~~

Get a named adapter to an interface for an object.  Returns an adapter
that can adapt object to interface.  If a matching adapter cannot be
found, raises ``ComponentLookupError`` .

 - Location: ``zope.interface``

 - Signature: `getAdapter(object, interface=Interface, name=u'', context=None)`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface

  >>> class IDesk(Interface):
  ...     """A frontdesk will register object's details"""
  ...
  ...     def register():
  ...         """Register object's details"""
  ...

  >>> from zope.interface import implements
  >>> from zope.component import adapts

  >>> class FrontDeskNG(object):
  ...
  ...     implements(IDesk)
  ...     adapts(IGuest)
  ...
  ...     def __init__(self, guest):
  ...         self.guest = guest
  ...
  ...     def register(self):
  ...         next_id = get_next_id()
  ...         bookings_db[next_id] = {
  ...         'name': guest.name,
  ...         'place': guest.place,
  ...         'phone': guest.phone
  ...         }

  >>> class Guest(object):
  ...
  ...     implements(IGuest)
  ...
  ...     def __init__(self, name, place):
  ...         self.name = name
  ...         self.place = place

  >>> jack = Guest("Jack", "Bangalore")
  >>> jack_frontdesk = FrontDeskNG(jack)

  >>> IDesk.providedBy(jack_frontdesk)
  True

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()
  >>> gsm.registerAdapter(FrontDeskNG,
  ...                     (IGuest,), IDesk, 'ng')

  >>> getAdapter(jack, IDesk, 'ng') #doctest: +ELLIPSIS
  <FrontDeskNG object at ...>


getAdapterInContext
~~~~~~~~~~~~~~~~~~~

Instead of this function, use `context` argument of `getAdapter`_
function.

 - Location: ``zope.component``

 - Signature: `getAdapterInContext(object, interface, context)`

 - See also: `queryAdapterInContext`_

Example::

  >>> from zope.component.globalregistry import BaseGlobalComponents
  >>> from zope.component import IComponentLookup
  >>> sm = BaseGlobalComponents()

  >>> class Context(object):
  ...     def __init__(self, sm):
  ...         self.sm = sm
  ...     def __conform__(self, interface):
  ...         if interface.isOrExtends(IComponentLookup):
  ...             return self.sm

  >>> context = Context(sm)

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface

  >>> class IDesk(Interface):
  ...     """A frontdesk will register object's details"""
  ...
  ...     def register():
  ...         """Register object's details"""
  ...

  >>> from zope.interface import implements
  >>> from zope.component import adapts

  >>> class FrontDeskNG(object):
  ...
  ...     implements(IDesk)
  ...     adapts(IGuest)
  ...
  ...     def __init__(self, guest):
  ...         self.guest = guest
  ...
  ...     def register(self):
  ...         next_id = get_next_id()
  ...         bookings_db[next_id] = {
  ...         'name': guest.name,
  ...         'place': guest.place,
  ...         'phone': guest.phone
  ...         }

  >>> class Guest(object):
  ...
  ...     implements(IGuest)
  ...
  ...     def __init__(self, name, place):
  ...         self.name = name
  ...         self.place = place

  >>> jack = Guest("Jack", "Bangalore")
  >>> jack_frontdesk = FrontDeskNG(jack)

  >>> IDesk.providedBy(jack_frontdesk)
  True

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()
  >>> sm.registerAdapter(FrontDeskNG,
  ...                     (IGuest,), IDesk)

  >>> from zope.component import getAdapterInContext

  >>> getAdapterInContext(jack, IDesk, sm) #doctest: +ELLIPSIS
  <FrontDeskNG object at ...>


getAdapters
~~~~~~~~~~~

Look for all matching adapters to a provided interface for objects.
Return a list of adapters that match. If an adapter is named, only the
most specific adapter of a given name is returned.

 - Location: ``zope.component``

 - Signature: `getAdapters(objects, provided, context=None)`

Example::

  >>> from zope.interface import implements
  >>> from zope.component import adapts

  >>> class FrontDeskNG(object):
  ...
  ...     implements(IDesk)
  ...     adapts(IGuest)
  ...
  ...     def __init__(self, guest):
  ...         self.guest = guest
  ...
  ...     def register(self):
  ...         next_id = get_next_id()
  ...         bookings_db[next_id] = {
  ...         'name': guest.name,
  ...         'place': guest.place,
  ...         'phone': guest.phone
  ...         }

  >>> jack = Guest("Jack", "Bangalore")
  >>> jack_frontdesk = FrontDeskNG(jack)

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> gsm.registerAdapter(FrontDeskNG, name='ng')

  >>> from zope.component import getAdapters
  >>> list(getAdapters((jack,), IDesk)) #doctest: +ELLIPSIS
  [(u'ng', <FrontDeskNG object at ...>)]


getAllUtilitiesRegisteredFor
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Return all registered utilities for an interface.  This includes
overridden utilities.  The returned value is an iterable of utility
instances.

 - Location: ``zope.component``

 - Signature: `getAllUtilitiesRegisteredFor(interface)`

Example::

  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IGreeter(Interface):
  ...     def greet(name):
  ...         "say hello"

  >>> class Greeter(object):
  ...
  ...     implements(IGreeter)
  ...
  ...     def greet(self, name):
  ...         print "Hello", name

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> greet = Greeter()
  >>> gsm.registerUtility(greet, IGreeter)

  >>> from zope.component import getAllUtilitiesRegisteredFor

  >>> getAllUtilitiesRegisteredFor(IGreeter) #doctest: +ELLIPSIS
  [<Greeter object at ...>]


getFactoriesFor
~~~~~~~~~~~~~~~

Return a tuple (name, factory) of registered factories that create
objects which implement the given interface.

 - Location: ``zope.component``

 - Signature: `getFactoriesFor(interface, context=None)`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IDatabase(Interface):
  ...
  ...     def getConnection():
  ...         """Return connection object"""

  >>> class FakeDb(object):
  ...
  ...     implements(IDatabase)
  ...
  ...     def getConnection(self):
  ...         return "connection"

  >>> from zope.component.factory import Factory

  >>> factory = Factory(FakeDb, 'FakeDb')

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> from zope.component.interfaces import IFactory
  >>> gsm.registerUtility(factory, IFactory, 'fakedb')

  >>> from zope.component import getFactoriesFor

  >>> list(getFactoriesFor(IDatabase))
  [(u'fakedb', <Factory for <class 'FakeDb'>>)]


getFactoryInterfaces
~~~~~~~~~~~~~~~~~~~~

Get interfaces implemented by a factory.  Finds the factory of the
given name that is nearest to the context, and returns the interface
or interface tuple that object instances created by the named factory
will implement.

 - Location: ``zope.component``

 - Signature: `getFactoryInterfaces(name, context=None)`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IDatabase(Interface):
  ...
  ...     def getConnection():
  ...         """Return connection object"""

  >>> class FakeDb(object):
  ...
  ...     implements(IDatabase)
  ...
  ...     def getConnection(self):
  ...         return "connection"

  >>> from zope.component.factory import Factory

  >>> factory = Factory(FakeDb, 'FakeDb')

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> from zope.component.interfaces import IFactory
  >>> gsm.registerUtility(factory, IFactory, 'fakedb')

  >>> from zope.component import getFactoryInterfaces

  >>> getFactoryInterfaces('fakedb')
  <implementedBy __builtin__.FakeDb>


getGlobalSiteManager
~~~~~~~~~~~~~~~~~~~~

Return the global site manager.  This function should never fail and
always return an object that provides `IGlobalSiteManager`

 - Location: ``zope.component``

 - Signature: `getGlobalSiteManager()`

Example::

  >>> from zope.component import getGlobalSiteManager
  >>> from zope.component import globalSiteManager
  >>> gsm = getGlobalSiteManager()
  >>> gsm is globalSiteManager
  True


getMultiAdapter
~~~~~~~~~~~~~~~

Look for a multi-adapter to an interface for an objects.  Returns a
multi-adapter that can adapt objects to interface.  If a matching
adapter cannot be found, raises ComponentLookupError.  The name
consisting of an empty string is reserved for unnamed adapters. The
unnamed adapter methods will often call the named adapter methods with
an empty string for a name.

 - Location: ``zope.component``

 - Signature: `getMultiAdapter(objects, interface=Interface, name='',
   context=None)`

 - See also: `queryMultiAdapter`_

Example::

  >>> from zope.interface import Interface
  >>> from zope.interface import implements
  >>> from zope.component import adapts

  >>> class IAdapteeOne(Interface):
  ...     pass

  >>> class IAdapteeTwo(Interface):
  ...     pass

  >>> class IFunctionality(Interface):
  ...     pass

  >>> class MyFunctionality(object):
  ...     implements(IFunctionality)
  ...     adapts(IAdapteeOne, IAdapteeTwo)
  ...
  ...     def __init__(self, one, two):
  ...         self.one = one
  ...         self.two = two

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> gsm.registerAdapter(MyFunctionality)

  >>> class One(object):
  ...     implements(IAdapteeOne)

  >>> class Two(object):
  ...     implements(IAdapteeTwo)

  >>> one = One()
  >>> two = Two()

  >>> from zope.component import getMultiAdapter

  >>> getMultiAdapter((one,two), IFunctionality) #doctest: +ELLIPSIS
  <MyFunctionality object at ...>

  >>> myfunctionality = getMultiAdapter((one,two), IFunctionality)
  >>> myfunctionality.one #doctest: +ELLIPSIS
  <One object at ...>
  >>> myfunctionality.two #doctest: +ELLIPSIS
  <Two object at ...>


getSiteManager
~~~~~~~~~~~~~~

Get the nearest site manager in the given context.  If `context` is
`None`, return the global site manager.  If the `context` is not
`None`, it is expected that an adapter from the `context` to
`IComponentLookup` can be found.  If no adapter is found, a
`ComponentLookupError` is raised.

 - Location: ``zope.component``

 - Signature: `getSiteManager(context=None)`

Example 1::

  >>> from zope.component.globalregistry import BaseGlobalComponents
  >>> from zope.component import IComponentLookup
  >>> sm = BaseGlobalComponents()

  >>> class Context(object):
  ...     def __init__(self, sm):
  ...         self.sm = sm
  ...     def __conform__(self, interface):
  ...         if interface.isOrExtends(IComponentLookup):
  ...             return self.sm

  >>> context = Context(sm)

  >>> from zope.component import getSiteManager

  >>> lsm = getSiteManager(context)
  >>> lsm is sm
  True

Example 2::

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> sm = getSiteManager()
  >>> gsm is sm
  True


getUtilitiesFor
~~~~~~~~~~~~~~~

Look up the registered utilities that provide an interface.  Returns
an iterable of name-utility pairs.

 - Location: ``zope.component``

 - Signature: `getUtilitiesFor(interface)`

Example::

  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IGreeter(Interface):
  ...     def greet(name):
  ...         "say hello"

  >>> class Greeter(object):
  ...
  ...     implements(IGreeter)
  ...
  ...     def greet(self, name):
  ...         print "Hello", name

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> greet = Greeter()
  >>> gsm.registerUtility(greet, IGreeter)

  >>> from zope.component import getUtilitiesFor

  >>> list(getUtilitiesFor(IGreeter)) #doctest: +ELLIPSIS
  [(u'', <Greeter object at ...>)]


getUtility
~~~~~~~~~~

Get the utility that provides interface.  Returns the nearest utility
to the context that implements the specified interface.  If one is not
found, raises ``ComponentLookupError``.

 - Location: ``zope.component``

 - Signature: `getUtility(interface, name='', context=None)`

Example::

  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IGreeter(Interface):
  ...     def greet(name):
  ...         "say hello"

  >>> class Greeter(object):
  ...
  ...     implements(IGreeter)
  ...
  ...     def greet(self, name):
  ...         return "Hello " + name

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> greet = Greeter()
  >>> gsm.registerUtility(greet, IGreeter)

  >>> from zope.component import getUtility

  >>> getUtility(IGreeter).greet('Jack')
  'Hello Jack'


handle
~~~~~~

Call all of the handlers for the given objects.  Handlers are
subscription adapter factories that don't produce anything.  They do
all of their work when called.  Handlers are typically used to handle
events.

 - Location: ``zope.component``

 - Signature: `handle(*objects)`

Example::

  >>> import datetime

  >>> def documentCreated(event):
  ...     event.doc.created = datetime.datetime.utcnow()

  >>> from zope.interface import Interface
  >>> from zope.interface import Attribute
  >>> from zope.interface import implements

  >>> class IDocumentCreated(Interface):
  ...     doc = Attribute("The document that was created")

  >>> class DocumentCreated(object):
  ...     implements(IDocumentCreated)
  ...
  ...     def __init__(self, doc):
  ...         self.doc = doc


  >>> def documentCreated(event):
  ...     event.doc.created = datetime.datetime.utcnow()

  >>> from zope.component import adapter

  >>> @adapter(IDocumentCreated)
  ... def documentCreated(event):
  ...     event.doc.created = datetime.datetime.utcnow()


  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> gsm.registerHandler(documentCreated)

  >>> from zope.component import handle

  >>> handle(DocumentCreated(doc))
  >>> doc.created.__class__.__name__
  'datetime'


implementedBy
~~~~~~~~~~~~~

Return the interfaces implemented for a class' instances.

 - Location: ``zope.interface``

 - Signature: `implementedBy(class_)`

Example 1::

  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IGreeter(Interface):
  ...     def greet(name):
  ...         "say hello"

  >>> class Greeter(object):
  ...
  ...     implements(IGreeter)
  ...
  ...     def greet(self, name):
  ...         print "Hello", name

  >>> from zope.interface import implementedBy
  >>> implementedBy(Greeter)
  <implementedBy __builtin__.Greeter>

Example 2::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IPerson(Interface):
  ...     name = Attribute("Name of person")

  >>> class ISpecial(Interface):
  ...     pass

  >>> class Person(object):
  ...     implements(IPerson)
  ...     name = u""

  >>> from zope.interface import classImplements
  >>> classImplements(Person, ISpecial)

  >>> from zope.interface import implementedBy

  To get a list of all interfaces implemented by that class::

  >>> [x.__name__ for x in implementedBy(Person)]
  ['IPerson', 'ISpecial']


implementer
~~~~~~~~~~~

Create a decorator for declaring interfaces implemented by a factory.
A callable is returned that makes an implements declaration on objects
passed to it.

 - Location: ``zope.interface``

 - Signature: `implementer(*interfaces)`

Example::

  >>> from zope.interface import implementer
  >>> class IFoo(Interface):
  ...     pass
  >>> class Foo(object):
  ...     implements(IFoo)

  >>> @implementer(IFoo)
  ... def foocreator():
  ...     foo = Foo()
  ...     return foo
  >>> list(implementedBy(foocreator))
  [<InterfaceClass __builtin__.IFoo>]


implements
~~~~~~~~~~

Declare interfaces implemented by instances of a class This function
is called in a class definition.  The arguments are one or more
interfaces.  The interfaces given are added to any interfaces
previously declared.  Previous declarations include declarations for
base classes unless implementsOnly was used.

 - Location: ``zope.interface``

 - Signature: `implements(*interfaces)`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IPerson(Interface):
  ...
  ...     name = Attribute("Name of person")

  >>> class Person(object):
  ...
  ...     implements(IPerson)
  ...     name = u""

  >>> jack = Person()
  >>> jack.name = "Jack"

  You can test it like this:

  >>> from zope.interface import providedBy
  >>> IPerson in providedBy(jack)
  True


implementsOnly
~~~~~~~~~~~~~~

Declare the only interfaces implemented by instances of a class.  This
function is called in a class definition.  The arguments are one or
more interfaces.  Previous declarations including declarations for
base classes are overridden.

 - Location: ``zope.interface``

 - Signature: `implementsOnly(*interfaces)`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface
  >>> from zope.interface import implements
  >>> from zope.interface import implementsOnly

  >>> class IPerson(Interface):
  ...
  ...     name = Attribute("Name of person")

  >>> class IStudent(Interface):
  ...
  ...     college = Attribute("Name of college")

  >>> class Person(object):
  ...
  ...     implements(IPerson)
  ...     name = u""

  >>> class NewPerson(Person):
  ...     implementsOnly(IStudent)
  ...     college = u""

  >>> jack = NewPerson()
  >>> jack.college = "New College"

  You can test it like this:

  >>> from zope.interface import providedBy
  >>> IPerson in providedBy(jack)
  False
  >>> IStudent in providedBy(jack)
  True


Interface
~~~~~~~~~

Using this class, you can define an interface.  To define an
interface, just inherit from ``Interface`` class.

 - Location: ``zope.interface``

 - Signature: `Interface(name, doc='')`

Example 1::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface

  >>> class IPerson(Interface):
  ...
  ...     name = Attribute("Name of person")
  ...     email = Attribute("Email Address")


Example 2::

  >>> from zope.interface import Interface

  >>> class IHost(Interface):
  ...
  ...     def goodmorning(guest):
  ...         """Say good morning to guest"""


moduleProvides
~~~~~~~~~~~~~~

Declare interfaces provided by a module.  This function is used in a
module definition.  The arguments are one or more interfaces.  The
given interfaces are used to create the module's direct-object
interface specification.  An error will be raised if the module
already has an interface specification.  In other words, it is an
error to call this function more than once in a module definition.

This function is provided for convenience.  It provides a more
convenient way to call ``directlyProvides`` for a module.

 - Location: ``zope.interface``

 - Signature: `moduleProvides(*interfaces)`

 - See also: `directlyProvides`_

You can see an example usage in `zope.component` source itself.  The
`__init__.py` file has a statement like this::

  moduleProvides(IComponentArchitecture,
                 IComponentRegistrationConvenience)

So, the `zope.component` provides two interfaces:
`IComponentArchitecture` and `IComponentRegistrationConvenience`.


noLongerProvides
~~~~~~~~~~~~~~~~

Remove an interface from the list of an object's directly provided
interfaces.

 - Location: ``zope.interface``

 - Signature: `noLongerProvides(object, interface)`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface
  >>> from zope.interface import implements
  >>> from zope.interface import classImplements

  >>> class IPerson(Interface):
  ...
  ...     name = Attribute("Name of person")

  >>> class IStudent(Interface):
  ...
  ...     college = Attribute("Name of college")

  >>> class Person(object):
  ...
  ...     implements(IPerson)
  ...     name = u""

  >>> jack = Person()
  >>> jack.name = "Jack"
  >>> jack.college = "New College"
  >>> directlyProvides(jack, IStudent)

  You can test it like this:

  >>> from zope.interface import providedBy
  >>> IPerson in providedBy(jack)
  True
  >>> IStudent in providedBy(jack)
  True
  >>> from zope.interface import noLongerProvides
  >>> noLongerProvides(jack, IStudent)
  >>> IPerson in providedBy(jack)
  True
  >>> IStudent in providedBy(jack)
  False


provideAdapter
~~~~~~~~~~~~~~

It is recommended to use `registerAdapter`_ .


provideHandler
~~~~~~~~~~~~~~

It is recommended to use `registerHandler`_ .


provideSubscriptionAdapter
~~~~~~~~~~~~~~~~~~~~~~~~~~

It is recommended to use `registerSubscriptionAdapter`_ .


provideUtility
~~~~~~~~~~~~~~

It is recommended to use `registerUtility`_ .


providedBy
~~~~~~~~~~

Test whether the interface is implemented by the object.  Return true
if the object asserts that it implements the interface, including
asserting that it implements an extended interface.

 - Location: ``zope.interface``

 - Signature: `providedBy(object)`

Example 1::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IPerson(Interface):
  ...
  ...     name = Attribute("Name of person")

  >>> class Person(object):
  ...
  ...     implements(IPerson)
  ...     name = u""

  >>> jack = Person()
  >>> jack.name = "Jack"

  You can test it like this:

  >>> from zope.interface import providedBy
  >>> IPerson in providedBy(jack)
  True

Example 2::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IPerson(Interface):
  ...     name = Attribute("Name of person")

  >>> class ISpecial(Interface):
  ...     pass

  >>> class Person(object):
  ...     implements(IPerson)
  ...     name = u""

  >>> from zope.interface import classImplements
  >>> classImplements(Person, ISpecial)
  >>> from zope.interface import providedBy
  >>> jack = Person()
  >>> jack.name = "Jack"

  To get a list of all interfaces provided by that object::

  >>> [x.__name__ for x in providedBy(jack)]
  ['IPerson', 'ISpecial']


queryAdapter
~~~~~~~~~~~~

Look for a named adapter to an interface for an object.  Returns an
adapter that can adapt object to interface.  If a matching adapter
cannot be found, returns the default.

 - Location: ``zope.component``

 - Signature: `queryAdapter(object, interface=Interface, name=u'',
   default=None, context=None)`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface

  >>> class IDesk(Interface):
  ...     """A frontdesk will register object's details"""
  ...
  ...     def register():
  ...         """Register object's details"""
  ...

  >>> from zope.interface import implements
  >>> from zope.component import adapts

  >>> class FrontDeskNG(object):
  ...
  ...     implements(IDesk)
  ...     adapts(IGuest)
  ...
  ...     def __init__(self, guest):
  ...         self.guest = guest
  ...
  ...     def register(self):
  ...         next_id = get_next_id()
  ...         bookings_db[next_id] = {
  ...         'name': guest.name,
  ...         'place': guest.place,
  ...         'phone': guest.phone
  ...         }

  >>> class Guest(object):
  ...
  ...     implements(IGuest)
  ...
  ...     def __init__(self, name, place):
  ...         self.name = name
  ...         self.place = place

  >>> jack = Guest("Jack", "Bangalore")
  >>> jack_frontdesk = FrontDeskNG(jack)

  >>> IDesk.providedBy(jack_frontdesk)
  True

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()
  >>> gsm.registerAdapter(FrontDeskNG,
  ...                     (IGuest,), IDesk, 'ng')

  >>> queryAdapter(jack, IDesk, 'ng') #doctest: +ELLIPSIS
  <FrontDeskNG object at ...>


queryAdapterInContext
~~~~~~~~~~~~~~~~~~~~~

Look for a special adapter to an interface for an object.

NOTE: This method should only be used if a custom context needs to be
provided to provide custom component lookup. Otherwise, call the
interface, as in::

  interface(object, default)

Returns an adapter that can adapt object to interface.  If a matching
adapter cannot be found, returns the default.

Context is adapted to IServiceService, and this adapter's 'Adapters'
service is used.

If the object has a __conform__ method, this method will be called
with the requested interface.  If the method returns a non-None value,
that value will be returned. Otherwise, if the object already
implements the interface, the object will be returned.

 - Location: ``zope.component``

 - Signature: `queryAdapterInContext(object, interface, context,
   default=None)`

 - See also: `getAdapterInContext`_

Example::

  >>> from zope.component.globalregistry import BaseGlobalComponents
  >>> from zope.component import IComponentLookup
  >>> sm = BaseGlobalComponents()

  >>> class Context(object):
  ...     def __init__(self, sm):
  ...         self.sm = sm
  ...     def __conform__(self, interface):
  ...         if interface.isOrExtends(IComponentLookup):
  ...             return self.sm

  >>> context = Context(sm)

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface

  >>> class IDesk(Interface):
  ...     """A frontdesk will register object's details"""
  ...
  ...     def register():
  ...         """Register object's details"""
  ...

  >>> from zope.interface import implements
  >>> from zope.component import adapts

  >>> class FrontDeskNG(object):
  ...
  ...     implements(IDesk)
  ...     adapts(IGuest)
  ...
  ...     def __init__(self, guest):
  ...         self.guest = guest
  ...
  ...     def register(self):
  ...         next_id = get_next_id()
  ...         bookings_db[next_id] = {
  ...         'name': guest.name,
  ...         'place': guest.place,
  ...         'phone': guest.phone
  ...         }

  >>> class Guest(object):
  ...
  ...     implements(IGuest)
  ...
  ...     def __init__(self, name, place):
  ...         self.name = name
  ...         self.place = place

  >>> jack = Guest("Jack", "Bangalore")
  >>> jack_frontdesk = FrontDeskNG(jack)

  >>> IDesk.providedBy(jack_frontdesk)
  True

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()
  >>> sm.registerAdapter(FrontDeskNG,
  ...                     (IGuest,), IDesk)

  >>> from zope.component import queryAdapterInContext

  >>> queryAdapterInContext(jack, IDesk, sm) #doctest: +ELLIPSIS
  <FrontDeskNG object at ...>


queryMultiAdapter
~~~~~~~~~~~~~~~~~

Look for a multi-adapter to an interface for objects.  Returns a
multi-adapter that can adapt objects to interface.  If a matching
adapter cannot be found, returns the default.  The name consisting of
an empty string is reserved for unnamed adapters.  The unnamed adapter
methods will often call the named adapter methods with an empty string
for a name.

 - Location: ``zope.component``

 - Signature: `queryMultiAdapter(objects, interface=Interface,
   name=u'', default=None, context=None)`

 - See also: `getMultiAdapter`_

Example::

  >>> from zope.interface import Interface
  >>> from zope.interface import implements
  >>> from zope.component import adapts

  >>> class IAdapteeOne(Interface):
  ...     pass

  >>> class IAdapteeTwo(Interface):
  ...     pass

  >>> class IFunctionality(Interface):
  ...     pass

  >>> class MyFunctionality(object):
  ...     implements(IFunctionality)
  ...     adapts(IAdapteeOne, IAdapteeTwo)
  ...
  ...     def __init__(self, one, two):
  ...         self.one = one
  ...         self.two = two

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> gsm.registerAdapter(MyFunctionality)

  >>> class One(object):
  ...     implements(IAdapteeOne)

  >>> class Two(object):
  ...     implements(IAdapteeTwo)

  >>> one = One()
  >>> two = Two()

  >>> from zope.component import queryMultiAdapter

  >>> getMultiAdapter((one,two), IFunctionality) #doctest: +ELLIPSIS
  <MyFunctionality object at ...>

  >>> myfunctionality = queryMultiAdapter((one,two), IFunctionality)
  >>> myfunctionality.one #doctest: +ELLIPSIS
  <One object at ...>
  >>> myfunctionality.two #doctest: +ELLIPSIS
  <Two object at ...>


queryUtility
~~~~~~~~~~~~

This function is used to look up a utility that provides an interface.
If one is not found, returns default.

 - Location: ``zope.component``

 - Signature: `queryUtility(interface, name='', default=None)`

Example::

  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IGreeter(Interface):
  ...     def greet(name):
  ...         "say hello"

  >>> class Greeter(object):
  ...
  ...     implements(IGreeter)
  ...
  ...     def greet(self, name):
  ...         return "Hello " + name

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> greet = Greeter()
  >>> gsm.registerUtility(greet, IGreeter)

  >>> from zope.component import queryUtility

  >>> queryUtility(IGreeter).greet('Jack')
  'Hello Jack'


registerAdapter
~~~~~~~~~~~~~~~

This function is used to register an adapter factory.

 - Location: ``zope.component - IComponentRegistry``

 - Signature: `registerAdapter(factory, required=None, provided=None,
   name=u'', info=u'')`

 - See also: `unregisterAdapter`_

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface

  >>> class IDesk(Interface):
  ...     """A frontdesk will register object's details"""
  ...
  ...     def register():
  ...         """Register object's details"""
  ...

  >>> from zope.interface import implements
  >>> from zope.component import adapts

  >>> class FrontDeskNG(object):
  ...
  ...     implements(IDesk)
  ...     adapts(IGuest)
  ...
  ...     def __init__(self, guest):
  ...         self.guest = guest
  ...
  ...     def register(self):
  ...         next_id = get_next_id()
  ...         bookings_db[next_id] = {
  ...         'name': guest.name,
  ...         'place': guest.place,
  ...         'phone': guest.phone
  ...         }

  >>> class Guest(object):
  ...
  ...     implements(IGuest)
  ...
  ...     def __init__(self, name, place):
  ...         self.name = name
  ...         self.place = place

  >>> jack = Guest("Jack", "Bangalore")
  >>> jack_frontdesk = FrontDeskNG(jack)

  >>> IDesk.providedBy(jack_frontdesk)
  True

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()
  >>> gsm.registerAdapter(FrontDeskNG,
  ...                     (IGuest,), IDesk, 'ng')

  You can test it like this:

  >>> queryAdapter(jack, IDesk, 'ng') #doctest: +ELLIPSIS
  <FrontDeskNG object at ...>


registeredAdapters
~~~~~~~~~~~~~~~~~~

Return an iterable of `IAdapterRegistrations`.  These registrations
describe the current adapter registrations in the object.

 - Location: ``zope.component - IComponentRegistry``

 - Signature: `registeredAdapters()`

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface

  >>> class IDesk(Interface):
  ...     """A frontdesk will register object's details"""
  ...
  ...     def register():
  ...         """Register object's details"""
  ...

  >>> from zope.interface import implements
  >>> from zope.component import adapts

  >>> class FrontDeskNG(object):
  ...
  ...     implements(IDesk)
  ...     adapts(IGuest)
  ...
  ...     def __init__(self, guest):
  ...         self.guest = guest
  ...
  ...     def register(self):
  ...         next_id = get_next_id()
  ...         bookings_db[next_id] = {
  ...         'name': guest.name,
  ...         'place': guest.place,
  ...         'phone': guest.phone
  ...         }

  >>> class Guest(object):
  ...
  ...     implements(IGuest)
  ...
  ...     def __init__(self, name, place):
  ...         self.name = name
  ...         self.place = place

  >>> jack = Guest("Jack", "Bangalore")
  >>> jack_frontdesk = FrontDeskNG(jack)

  >>> IDesk.providedBy(jack_frontdesk)
  True

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()
  >>> gsm.registerAdapter(FrontDeskNG,
  ...                     (IGuest,), IDesk, 'ng2')


  >>> reg_adapter = list(gsm.registeredAdapters())
  >>> 'ng2' in [x.name for x in reg_adapter]
  True


registeredHandlers
~~~~~~~~~~~~~~~~~~

Return an iterable of `IHandlerRegistrations`.  These registrations
describe the current handler registrations in the object.

 - Location: ``zope.component - IComponentRegistry``

 - Signature: `registeredHandlers()`

Example::

  >>> import datetime

  >>> def documentCreated(event):
  ...     event.doc.created = datetime.datetime.utcnow()

  >>> from zope.interface import Interface
  >>> from zope.interface import Attribute
  >>> from zope.interface import implements

  >>> class IDocumentCreated(Interface):
  ...     doc = Attribute("The document that was created")

  >>> class DocumentCreated(object):
  ...     implements(IDocumentCreated)
  ...
  ...     def __init__(self, doc):
  ...         self.doc = doc


  >>> def documentCreated(event):
  ...     event.doc.created = datetime.datetime.utcnow()

  >>> from zope.component import adapter

  >>> @adapter(IDocumentCreated)
  ... def documentCreated(event):
  ...     event.doc.created = datetime.datetime.utcnow()


  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> gsm.registerHandler(documentCreated, info='ng3')

  >>> reg_adapter = list(gsm.registeredHandlers())
  >>> 'ng3' in [x.info for x in reg_adapter]
  True

  >>> gsm.registerHandler(documentCreated, name='ng4')
  Traceback (most recent call last):
  ...
  TypeError: Named handlers are not yet supported


registeredSubscriptionAdapters
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Return an iterable of `ISubscriptionAdapterRegistrations`.  These
registrations describe the current subscription adapter registrations
in the object.

 - Location: ``zope.component - IComponentRegistry``

 - Signature: `registeredSubscriptionAdapters()`

Example::

  >>> from zope.interface import Interface
  >>> from zope.interface import Attribute
  >>> from zope.interface import implements

  >>> class IValidate(Interface):
  ...     def validate(ob):
  ...         """Determine whether the object is valid
  ...
  ...         Return a string describing a validation problem.
  ...         An empty string is returned to indicate that the
  ...         object is valid.
  ...         """

  >>> class IDocument(Interface):
  ...     summary = Attribute("Document summary")
  ...     body = Attribute("Document text")

  >>> class Document(object):
  ...     implements(IDocument)
  ...     def __init__(self, summary, body):
  ...         self.summary, self.body = summary, body

  >>> from zope.component import adapts

  >>> class AdequateLength(object):
  ...
  ...     adapts(IDocument)
  ...     implements(IValidate)
  ...
  ...     def __init__(self, doc):
  ...         self.doc = doc
  ...
  ...     def validate(self):
  ...         if len(self.doc.body) < 1000:
  ...             return 'too short'
  ...         else:
  ...             return ''

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> gsm.registerSubscriptionAdapter(AdequateLength, info='ng4')

  >>> reg_adapter = list(gsm.registeredSubscriptionAdapters())
  >>> 'ng4' in [x.info for x in reg_adapter]
  True


registeredUtilities
~~~~~~~~~~~~~~~~~~~

This function return an iterable of `IUtilityRegistrations`.  These
registrations describe the current utility registrations in the
object.

 - Location: ``zope.component - IComponentRegistry``

 - Signature: `registeredUtilities()`

Example::

  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IGreeter(Interface):
  ...     def greet(name):
  ...         "say hello"

  >>> class Greeter(object):
  ...
  ...     implements(IGreeter)
  ...
  ...     def greet(self, name):
  ...         print "Hello", name

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> greet = Greeter()
  >>> gsm.registerUtility(greet, info='ng5')

  >>> reg_adapter = list(gsm.registeredUtilities())
  >>> 'ng5' in [x.info for x in reg_adapter]
  True


registerHandler
~~~~~~~~~~~~~~~

This function is used to register a handler.  A handler is a
subscriber that doesn't compute an adapter but performs some function
when called.

 - Location: ``zope.component - IComponentRegistry``

 - Signature: `registerHandler(handler, required=None, name=u'', info='')`

 - See also: `unregisterHandler`_

Note: In the current implementation of ``zope.component`` doesn't
support `name` attribute.

Example::

  >>> import datetime

  >>> def documentCreated(event):
  ...     event.doc.created = datetime.datetime.utcnow()

  >>> from zope.interface import Interface
  >>> from zope.interface import Attribute
  >>> from zope.interface import implements

  >>> class IDocumentCreated(Interface):
  ...     doc = Attribute("The document that was created")

  >>> class DocumentCreated(object):
  ...     implements(IDocumentCreated)
  ...
  ...     def __init__(self, doc):
  ...         self.doc = doc


  >>> def documentCreated(event):
  ...     event.doc.created = datetime.datetime.utcnow()

  >>> from zope.component import adapter

  >>> @adapter(IDocumentCreated)
  ... def documentCreated(event):
  ...     event.doc.created = datetime.datetime.utcnow()


  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> gsm.registerHandler(documentCreated)

  >>> from zope.component import handle

  >>> handle(DocumentCreated(doc))
  >>> doc.created.__class__.__name__
  'datetime'


registerSubscriptionAdapter
~~~~~~~~~~~~~~~~~~~~~~~~~~~

This function is used to register a subscriber factory.

 - Location: ``zope.component - IComponentRegistry``

 - Signature: `registerSubscriptionAdapter(factory, required=None,
   provides=None, name=u'', info='')`

 - See also: `unregisterSubscriptionAdapter`_

Example::

  >>> from zope.interface import Interface
  >>> from zope.interface import Attribute
  >>> from zope.interface import implements

  >>> class IValidate(Interface):
  ...     def validate(ob):
  ...         """Determine whether the object is valid
  ...
  ...         Return a string describing a validation problem.
  ...         An empty string is returned to indicate that the
  ...         object is valid.
  ...         """

  >>> class IDocument(Interface):
  ...     summary = Attribute("Document summary")
  ...     body = Attribute("Document text")

  >>> class Document(object):
  ...     implements(IDocument)
  ...     def __init__(self, summary, body):
  ...         self.summary, self.body = summary, body

  >>> from zope.component import adapts

  >>> class AdequateLength(object):
  ...
  ...     adapts(IDocument)
  ...     implements(IValidate)
  ...
  ...     def __init__(self, doc):
  ...         self.doc = doc
  ...
  ...     def validate(self):
  ...         if len(self.doc.body) < 1000:
  ...             return 'too short'
  ...         else:
  ...             return ''

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> gsm.registerSubscriptionAdapter(AdequateLength)


registerUtility
~~~~~~~~~~~~~~~

This function is used to register a utility.

 - Location: ``zope.component - IComponentRegistry``

 - Signature: `registerUtility(component, provided=None, name=u'',
   info=u'')`

 - See also: `unregisterUtility`_

Example::

  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IGreeter(Interface):
  ...     def greet(name):
  ...         "say hello"

  >>> class Greeter(object):
  ...
  ...     implements(IGreeter)
  ...
  ...     def greet(self, name):
  ...         print "Hello", name

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> greet = Greeter()
  >>> gsm.registerUtility(greet)


subscribers
~~~~~~~~~~~

This function is used to get subscribers.  Subscribers are returned
that provide the provided interface and that depend on and are
computed from the sequence of required objects.

 - Location: ``zope.component - IComponentRegistry``

 - Signature: `subscribers(required, provided, context=None)`

Example::

  >>> from zope.interface import Interface
  >>> from zope.interface import Attribute
  >>> from zope.interface import implements

  >>> class IValidate(Interface):
  ...     def validate(ob):
  ...         """Determine whether the object is valid
  ...
  ...         Return a string describing a validation problem.
  ...         An empty string is returned to indicate that the
  ...         object is valid.
  ...         """

  >>> class IDocument(Interface):
  ...     summary = Attribute("Document summary")
  ...     body = Attribute("Document text")

  >>> class Document(object):
  ...     implements(IDocument)
  ...     def __init__(self, summary, body):
  ...         self.summary, self.body = summary, body

  >>> from zope.component import adapts

  >>> class SingleLineSummary:
  ...     adapts(IDocument)
  ...     implements(IValidate)
  ...
  ...     def __init__(self, doc):
  ...         self.doc = doc
  ...
  ...     def validate(self):
  ...         if '\n' in self.doc.summary:
  ...             return 'Summary should only have one line'
  ...         else:
  ...             return ''

  >>> class AdequateLength(object):
  ...     adapts(IDocument)
  ...     implements(IValidate)
  ...
  ...     def __init__(self, doc):
  ...         self.doc = doc
  ...
  ...     def validate(self):
  ...         if len(self.doc.body) < 1000:
  ...             return 'too short'
  ...         else:
  ...             return ''

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> gsm.registerSubscriptionAdapter(SingleLineSummary)
  >>> gsm.registerSubscriptionAdapter(AdequateLength)

  >>> from zope.component import subscribers

  >>> doc = Document("A\nDocument", "blah")
  >>> [adapter.validate()
  ...  for adapter in subscribers([doc], IValidate)
  ...  if adapter.validate()]
  ['Summary should only have one line', 'too short']

  >>> doc = Document("A\nDocument", "blah" * 1000)
  >>> [adapter.validate()
  ...  for adapter in subscribers([doc], IValidate)
  ...  if adapter.validate()]
  ['Summary should only have one line']

  >>> doc = Document("A Document", "blah")
  >>> [adapter.validate()
  ...  for adapter in subscribers([doc], IValidate)
  ...  if adapter.validate()]
  ['too short']


unregisterAdapter
~~~~~~~~~~~~~~~~~

This function is used to unregister an adapter factory.  A boolean is
returned indicating whether the registry was changed.  If the given
component is None and there is no component registered, or if the
given component is not None and is not registered, then the function
returns False, otherwise it returns True.

 - Location: ``zope.component - IComponentRegistry``

 - Signature: `unregisterAdapter(factory=None, required=None,
   provided=None, name=u'')`

 - See also: `registerAdapter`_

Example::

  >>> from zope.interface import Attribute
  >>> from zope.interface import Interface

  >>> class IDesk(Interface):
  ...     """A frontdesk will register object's details"""
  ...
  ...     def register():
  ...         """Register object's details"""
  ...

  >>> from zope.interface import implements
  >>> from zope.component import adapts

  >>> class FrontDeskNG(object):
  ...
  ...     implements(IDesk)
  ...     adapts(IGuest)
  ...
  ...     def __init__(self, guest):
  ...         self.guest = guest
  ...
  ...     def register(self):
  ...         next_id = get_next_id()
  ...         bookings_db[next_id] = {
  ...         'name': guest.name,
  ...         'place': guest.place,
  ...         'phone': guest.phone
  ...         }

  >>> class Guest(object):
  ...
  ...     implements(IGuest)
  ...
  ...     def __init__(self, name, place):
  ...         self.name = name
  ...         self.place = place

  >>> jack = Guest("Jack", "Bangalore")
  >>> jack_frontdesk = FrontDeskNG(jack)

  >>> IDesk.providedBy(jack_frontdesk)
  True

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()
  >>> gsm.registerAdapter(FrontDeskNG,
  ...                     (IGuest,), IDesk, 'ng6')

  You can test it like this:

  >>> queryAdapter(jack, IDesk, 'ng6') #doctest: +ELLIPSIS
  <FrontDeskNG object at ...>

  Now unregister:

  >>> gsm.unregisterAdapter(FrontDeskNG, name='ng6')
  True

  After unregistration:

  >>> print queryAdapter(jack, IDesk, 'ng6')
  None


unregisterHandler
~~~~~~~~~~~~~~~~~

This function is used for unregistering a handler.  A handler is a
subscriber that doesn't compute an adapter but performs some function
when called.  A boolean is returned indicating whether the registry
was changed.

 - Location: ``zope.component - IComponentRegistry``

 - Signature: `unregisterHandler(handler=None, required=None,
   name=u'')`

 - See also: `registerHandler`_

Example::

  >>> from zope.interface import Interface
  >>> from zope.interface import Attribute
  >>> from zope.interface import implements

  >>> class IDocument(Interface):
  ...
  ...     summary = Attribute("Document summary")
  ...     body = Attribute("Document text")

  >>> class Document(object):
  ...
  ...     implements(IDocument)
  ...     def __init__(self, summary, body):
  ...         self.summary, self.body = summary, body

  >>> doc = Document("A\nDocument", "blah")

  >>> class IDocumentAccessed(Interface):
  ...     doc = Attribute("The document that was accessed")

  >>> class DocumentAccessed(object):
  ...     implements(IDocumentAccessed)
  ...
  ...     def __init__(self, doc):
  ...         self.doc = doc
  ...         self.doc.count = 0

  >>> from zope.component import adapter

  >>> @adapter(IDocumentAccessed)
  ... def documentAccessed(event):
  ...     event.doc.count = event.doc.count + 1

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> gsm.registerHandler(documentAccessed)

  >>> from zope.component import handle

  >>> handle(DocumentAccessed(doc))
  >>> doc.count
  1

  Now unregister:

  >>> gsm.unregisterHandler(documentAccessed)
  True

  After unregistration:

  >>> handle(DocumentAccessed(doc))
  >>> doc.count
  0


unregisterSubscriptionAdapter
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This function is used to unregister a subscriber factory.  A boolean
is returned indicating whether the registry was changed.  If the given
component is None and there is no component registered, or if the
given component is not None and is not registered, then the function
returns False, otherwise it returns True.

 - Location: ``zope.component - IComponentRegistry``

 - Signature: `unregisterSubscriptionAdapter(factory=None,
   required=None, provides=None, name=u'')`

 - See also: `registerSubscriptionAdapter`_

Example::

  >>> from zope.interface import Interface
  >>> from zope.interface import Attribute
  >>> from zope.interface import implements

  >>> class IValidate(Interface):
  ...     def validate(ob):
  ...         """Determine whether the object is valid
  ...
  ...         Return a string describing a validation problem.
  ...         An empty string is returned to indicate that the
  ...         object is valid.
  ...         """

  >>> class IDocument(Interface):
  ...     summary = Attribute("Document summary")
  ...     body = Attribute("Document text")

  >>> class Document(object):
  ...     implements(IDocument)
  ...     def __init__(self, summary, body):
  ...         self.summary, self.body = summary, body

  >>> from zope.component import adapts

  >>> class AdequateLength(object):
  ...
  ...     adapts(IDocument)
  ...     implements(IValidate)
  ...
  ...     def __init__(self, doc):
  ...         self.doc = doc
  ...
  ...     def validate(self):
  ...         if len(self.doc.body) < 1000:
  ...             return 'too short'
  ...         else:
  ...             return ''

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> gsm.registerSubscriptionAdapter(AdequateLength)

  >>> from zope.component import subscribers

  >>> doc = Document("A\nDocument", "blah")
  >>> [adapter.validate()
  ...  for adapter in subscribers([doc], IValidate)
  ...  if adapter.validate()]
  ['too short']

  Now unregister:

  >>> gsm.unregisterSubscriptionAdapter(AdequateLength)
  True

  After unregistration:

  >>> [adapter.validate()
  ...  for adapter in subscribers([doc], IValidate)
  ...  if adapter.validate()]
  []


unregisterUtility
~~~~~~~~~~~~~~~~~

This function is used for unregistering a utility.  A boolean is
returned indicating whether the registry was changed.  If the
given component is None and there is no component registered, or if
the given component is not None and is not registered, then the
function returns False, otherwise it returns True.

 - Location: ``zope.component - IComponentRegistry``

 - Signature: `unregisterUtility(component=None, provided=None,
   name=u'')`

 - See also: `registerUtility`_

Example::

  >>> from zope.interface import Interface
  >>> from zope.interface import implements

  >>> class IGreeter(Interface):
  ...     def greet(name):
  ...         "say hello"

  >>> class Greeter(object):
  ...
  ...     implements(IGreeter)
  ...
  ...     def greet(self, name):
  ...         return "Hello " + name

  >>> from zope.component import getGlobalSiteManager
  >>> gsm = getGlobalSiteManager()

  >>> greet = Greeter()
  >>> gsm.registerUtility(greet)

  >>> queryUtility(IGreeter).greet('Jack')
  'Hello Jack'

  Now unregister:

  >>> gsm.unregisterUtility(greet)
  True

  After unregistration:

  >>> print queryUtility(IGreeter)
  None