Exceptions could use some named attributes

[jamadden]

In our validation code using zope.schema fields, we have lots of things that look like this:

try:
    field.validate(value)
except WrongContainedType as e:
    if e.args and all(isinstance(SchemaNotProvided, v) for v in e.args[0]):
        # try to adapt
except WrongType as e:
    if len(e.args) == 3:
         expected_type = e.args[1]
          # try to adapt
except TooLong as e:
    if len(e.args) == 2:
        max_size = e.args[1]
        # Produce a pretty error message

All this argument parsing feels fragile.

With named attributes, we could document what exceptions are expected to have. We could use keyword arguments (with defaults) in the constructors to maintain compatibility with code that raises bare exceptions or uses different arguments.