The diplomatic path to building state machines in modern Python.

statesman is a library that provides an elegant and expressive API for implementing state machines in asynchronous Python 3.8+. It will negotiate with complexity on your behalf and broker a clear, concise agreement about how state is to be managed going forward.

• A lightweight, but fully featured implementation of the usual suspects in finite state machine libraries (states, events, transitions, actions).
• A declarative, simple API utilizing type hints, decorators, and enums.
• Provides a rich set of actions and callbacks for states and events. States support entry and exit actions, events have guard, before, on, and after actions. State machine wide callbacks are provided via overridable methods.
• Designed and built async native. Callbacks are dispatched asynchronously, making it easy to integrate with long running, event driven processes.
• Guard actions can cancel transition before any state changes are applied.
• Data can be modeled directly on the state machine subclass compliments of Pydantic.
• Events support the use of arbitrary associated parameter data that is made available to all actions. Parameters are matched against the signature of the receiving callable, enabling compartmentalization of concerns.
• Solid test coverage and documentation.

So... what's it look like? Glad you asked.

from typing import Optional, List
import statesman


class ProcessLifecycle(statesman.StateMachine):
    class States(statesman.StateEnum):
        starting = "Starting..."
        running = "Running..."
        stopping = "Stopping..."
        stopped = "Terminated."

    # Track state about the process we are running
    command: Optional[str] = None
    pid: Optional[int] = None
    logs: List[str] = []

    # initial state entry point
    @statesman.event(None, States.starting)
    async def start(self, command: str) -> None:
        """"Start a process."""
        self.command = command
        self.pid = 31337
        self.logs.clear()  # Flush logs between runs

    @statesman.event(source=States.starting, target=States.running)
    async def run(self, transition: statesman.Transition) -> None:
        """Mark the process as running."""
        self.logs.append(f"Process pid {self.pid} is now running (command=\"{self.command}\")")

    @statesman.event(source=States.running, target=States.stopping)
    async def stop(self) -> None:
        """Stop a running process."""
        self.logs.append(f"Shutting down pid {self.pid} (command=\"{self.command}\")")

    @statesman.event(source=States.stopping, target=States.stopped)
    async def terminate(self) -> None:
        """Terminate a running process."""
        self.logs.append(f"Terminated pid {self.pid} (\"{self.command}\")")
        self.command = None
        self.pid = None

    @statesman.enter_state(States.stopping)
    async def _print_status(self) -> None:
        print("Entering stopped status!")

    @statesman.after_event("run")
    async def _after_run(self) -> None:
        print("running...")

    async def after_transition(self, transition: statesman.Transition) -> None:
        if transition.event and transition.event.name == "stop":
            await self.terminate()


async def _examples():
    # Let's play.
    state_machine = ProcessLifecycle()
    await state_machine.start("ls -al")
    assert state_machine.command == "ls -al"
    assert state_machine.pid == 31337
    assert state_machine.state == ProcessLifecycle.States.starting

    await state_machine.run()
    assert state_machine.logs == ['Process pid 31337 is now running (command="ls -al")']

    await state_machine.stop()
    assert state_machine.logs == [
        'Process pid 31337 is now running (command="ls -al")',
        'Shutting down pid 31337 (command="ls -al")',
        'Terminated pid 31337 ("ls -al")',
    ]

    # Or start in a specific state
    state_machine = ProcessLifecycle(state=ProcessLifecycle.States.running)

    # Transition to a specific state
    await state_machine.enter_state(ProcessLifecycle.States.stopping)

    # Trigger an event
    await state_machine.trigger_event("stop", key="value")

States are defined as Python enum classes. The name of the enum item defines a symbolic name for the state and the value provides a human readable description. A class named States embedded within a state machine subclass is automatically bound to the state machine.

Events are declared using the event decorator and the define the source and target states of a transition. A source state of None defines an initial state transition.

Once a method is decorated as an event action, the original method body is attached to the new event as an on event action and the method is replaced with a implementation that triggers the newly created event.

Actions can be attached to events at declaration time or later on via the guard_event, before_event, on_event, and after_event decorators. Actions can likewise be attached to states via the enter_state and exit_state decorators.

There is an extensive API for working with the state machine and its components programmatically.

Statesman was developed because we couldn't find anything like it. While there is an embarassment of riches in the Python community with regard to FSM libraries, many have legacy ties back to the Python 2 era and address our first class requirements as add-ons rather than core functionality.

Our design goals were roughly:

• Utilize type hints extensively as both a documentation and design by contract tool to provide an expressive, readable API.
• Embrace asyncio as a first class citizen.
• Implement state machines as plain old Python objects with an easily understood method and input/output external API surface. Callers shouldn't need to know or care about FSM minutiae.
• Deliver an API closely aligned with the UML State Machine conceptual framework that most developers will have exposure to.
• Shun famous string values in favor of Python enum subclasses to facilitate type enforcement, refactoring, and IDE completions.
• Enable state machines to be modeled programmatically or declaratively.
• Provide robust support for modeling data within the state machine class itself and passing external data into transitions.
• Disallow implicit "magical" behaviors such as automatically creating states, events, and actions objects or dynamically defining/dispatching methods.

Ultimately, we really wanted something that would fit right in with our existing coding style and API aesthetics. Maybe statesman matches yours too.

Statesman executes actions during a transition in a defined order. The statesman.Transition class is a callable that is responsible for modeling a state change and coordinating its execution. The table below describes the order of operations performed when a transition is called.

Statesman is extensively covered with docstrings and automated tests. The proceding subsections present a non-exhaustive task oriented overview of the API. Check the docstrings and look through the tests if you don't find exactly what you are looking for.

Statesman considers a transition in which the source state is None to describe an initial state transition. There are a couple of ways to describe initial states within statesman:

import statesman


# Describe the initial state with `statesman.InitialState`
class StateMachine(statesman.StateMachine):
    class States(statesman.StateEnum):
        starting = 'Starting...'
        running = 'Running...'
        stopping = 'Stopping...'
        stopped = statesman.InitialState('Terminated.')

    @statesman.event(None, States.starting)
    def start(self) -> None:
        ...


async def _example() -> None:
    # Set at initialization time
    state_machine = StateMachine(state=StateMachine.States.stopping)

    # Enter a state directly
    state_machine = StateMachine()
    await state_machine.enter_state(StateMachine.States.running)

    # Via an event
    state_machine = StateMachine()
    await state_machine.start()

Each state machine instance has a state attribute that is the source of truth for the state machine. The state can be compared to statesman.State object instances or string values.

import statesman


class StateMachine(statesman.StateMachine):
    class States(statesman.StateEnum):
        starting = 'Starting...'
        running = 'Running...'
        stopping = 'Stopping...'
        stopped = 'Terminated.'


async def _example() -> None:
    state_machine = StateMachine(state=StateMachine.States.stopping)
    state_machine.state == StateMachine.States.stopping  # => True
    state_machine.state == "stopping"  # => True
    state_machine.state == StateMachine.States.running  # => False
    state_machine.state == "stopped"  # => False

States can be directly entered via the statesman.StateMachine.enter_state method. States can be referenced by name or by stateman.State object instance. When a state is entered directly, a transition is triggered between the source and target states:

import statesman


class StateMachine(statesman.HistoryMixin, statesman.StateMachine):
    class States(statesman.StateEnum):
        first = "1"
        second = "2"
        third = "3"


async def _example() -> None:
    state_machine = StateMachine()
    await state_machine.enter_state(StateMachine.States.first)
    await state_machine.enter_state(StateMachine.States.second)
    await state_machine.enter_state(StateMachine.States.third)

The type of transition performed is configurable (see below).

Note that enter_state should be used thoughtfully as it enables transitions that may not be expressible via events. Its behavior can also be constrained and customized.

There are three types of transitions that can be performed by statesman. The most common type is an external transition in which the machine moves between two distinct states. When a transition occurs in which the source and target states are the same, there two other possible modes: internal and self.

• statesman.Transition.Types.external: A transition in which the state is changed from one value to another.
• statesman.Transition.Types.internal: A transition in which the source and target states are the same but are not exited and reentered during the transition.
• statesman.Transition.Types.self: A transition in which the source and target states are the same and are exited and reentered during the transition.

To enable external consumers to interact with the state machine without being exposed to its implementation details, Statesman supports a flexible set of return values from transitions.

A default return type can be configured when defining an event via the decorators or programmatic interface. An explicit return value type can also be requested when a transition is dispatched through the statesman.StateMachine.enter_state or statesman.StateMachine.trigger_event methods.

Transitions can invoke an arbitrary number of actions and the desired return value semantics vary from case to case. Think about the API you wish to present to the developer and carefully consider if/how None and False values are utilized within the state machine.

The available return types are:

• bool: A boolean value that indicates if the transition completed successfully.
• object: An arbitrary output value returned by the transition.
• tuple: A tuple value containing a boolean and output object value.
• list: A list of results returned by all actions invoked by the transition.
• statesman.Transition: The transition object itself, containing all details about the transition.

Note that return types are configured as type arguments:

import statesman


class StateMachine(statesman.StateMachine):
    class States(statesman.StateEnum):
        starting = 'Starting'
        running = 'Running'
        stopping = 'Stopping'
        stopped = 'Stopped'

    @statesman.event(None, States.starting, return_type=bool)
    async def start(self) -> int:
        return 31337


async def _example() -> None:
    state_machine = await StateMachine.create()
    bool_result = await state_machine.trigger_event("start")
    int_result = await state_machine.start(return_type=object)
    transition = await state_machine.enter_state(
        StateMachine.States.stopped,
        return_type=statesman.Transition
    )
    print(
        f"Return Values: state_machine={state_machine}\n"
        f"bool_result={bool_result}\n"
        f"int_result={int_result}\n"
        f"transition={transition}"
    )

Events are typically defined using the statesman.event decorator. Each event has a source state and a target state. Typically these are distinct states and describe an event that triggers an external transition. When the source and target state are the same, the event describes an internal or self transition (see above for details).

The source and target states can be described by string name, enum member value, or the special sentinel values of statesman.StateEnum.__any__ or statesman.StateEnum.__active__. The __any__ sentinel describes a source state of any member of the state enumeration (but not None). The __active__ sentinel resolves dynamically to the currently active state of the state machine and is useful in cases where you want to define a reflexive internal or self transition that can be triggered from several states without duplicating logic.

Events can be triggered programmatically by name, method, or by calling a decorated event method.

import statesman


class StateMachine(statesman.StateMachine):
    class States(statesman.StateEnum):
        waiting = 'Waiting'
        running = 'Running'
        stopped = 'Stopped'
        aborted = 'Aborted'

    @statesman.event(None, States.waiting)
    async def start(self) -> None:
        ...

    @statesman.event(States.waiting, States.running)
    async def run(self) -> None:
        ...

    @statesman.event(States.running, States.stopped)
    async def stop(self) -> None:
        ...

    @statesman.event(States.__any__, States.aborted)
    async def abort(self) -> None:
        ...

    @statesman.event(
        States.__any__,
        States.__active__,
        type=statesman.Transition.Types.self
    )
    async def check(self) -> None:
        print("Exiting and reentering active state!")


async def _example() -> None:
    state_machine = await StateMachine.create()
    await state_machine.trigger_event("start")
    await state_machine.run()
    await state_machine.trigger_event(state_machine.stop)

States and Events support the attachment of an arbitrary number of actions. An action is an object that wraps a callable that called at a designated moment in the lifecycle of the state machine.

State objects support the following action types:

• statesman.Action.Types.entry: Called when a state is entered during a transition.
• statesman.Action.Types.exit: Called when a state is exited during a transition.

Event actions support the following action types:

• statesman.Action.Types.guard: Called to determine if the event can be executed.
• statesman.Action.Types.before: Called before the state transition described by the event is applied. The state of the machine is the source state.
• statesman.Action.Types.on: Called when the state transition described by the event is applied. The state of the machine is the target state.
• statesman.Action.Types.after: Called after the state transition described by the event has been applied. The state of the machine is the target state.

State and Event actions can be defined in several ways. The statesman.state and statesman.event decorators accept keyword arguments named after the action types that they support. These arguments support method object references, names, or callables (e.g., lambdas). Additionally, there are standalone decorators that enable a declarative style of action definition.

import random
import statesman


class StateMachine(statesman.StateMachine):
    class States(statesman.StateEnum):
        starting = 'Starting...'
        running = 'Running...'
        stopping = 'Stopping...'
        stopped = statesman.InitialState('Terminated.')

    @statesman.event(States.stopped, States.starting)
    async def start(self) -> None:
        ...

    @statesman.enter_state(States.starting)
    async def _announce_start(self) -> None:
        print("enter:starting")

    def _can_run(self) -> bool:
        return False

    @statesman.event(States.starting, States.running, guard=_can_run)
    async def run(self) -> None:
        ...

    @statesman.event(States.running, States.stopping)
    async def stop(self) -> None:
        ...

    @statesman.after_event(stop)
    async def _announce_stop(self) -> None:
        print("after:stop")

    @statesman.event(
        States.stopping,
        States.stopped,
        guard=lambda: random.choice([True, False])
    )
    async def terminate(self) -> None:
        ...

Guard callbacks and actions are handled differently from other behavioral hooks. Because guards can be used to cancel/reject an event, they are executed sequentially in the order that they were added to the state machine.

Upon encountering a failing guard that has returned False or raised an exception of type AssertionError, the state machine consults the guard_with behavior configured on the Config class nested within the state machine class.

There are three behaviors available for configuration via guard_with:

• statesman.Guard.silence (Default): The transition is aborted and an empty result set is returned.
• statesman.Guard.warning: The transition is aborted, an empty result set is returned, and a warning is logged.
• statesman.Guard.exception: The transition is aborted and a RuntimeError is raised.

import statesman


class StateMachine(statesman.StateMachine):
    class States(statesman.StateEnum):
        starting = 'Starting...'
        running = 'Running...'
        stopping = 'Stopping...'
        stopped = statesman.InitialState('Terminated.')

    class Config:
        guard_with = statesman.Guard.exception

Statesman is designed to model and manage arbitrary data that is bound to the current state of the state machine. Such data can be provided as positional and keyword arguments when a transition is triggered via the statesman.StateMachine.enter_state or statesman.StateMachine.trigger_event methods.

Statesman utilizes method argument list introspection and type-hinting to invoke callbacks and actions with the specific arguments that they are interested in. This facilitates cleaner encapsulation, separation of concerns, and maintainability by ensuring that each callback and method declares its expectations. The *args and **kwargs variadic arguments can be used as necessary.

import statesman


class StateMachine(statesman.StateMachine):
    class States(statesman.StateEnum):
        starting = 'Starting...'
        running = 'Running...'
        stopping = 'Stopping...'
        stopped = statesman.InitialState('Terminated.')

    @statesman.event(States.stopped, States.starting)
    async def start(self, process_name: str) -> None:
        ...

    @statesman.event(States.starting, States.running)
    async def run(self, uid: int, gid: int) -> None:
        ...

    @statesman.event(States.running, States.stopping)
    async def stop(self, *args, all: bool = False, **kwargs) -> None:
        ...


async def _example() -> None:
    state_machine = await StateMachine.create()
    await state_machine.start("servox")
    await state_machine.run(0, 31337)
    await state_machine.stop("one", "two", all=True, this="That")

The statesman.StateMachine class is typically subclassed. The base class provides inheritable method implementations of transition lifecycle events. Analogous functionality can be implemented through callbacks as through the State and Event actions except that perspective is rooted on the transition as opposed to the State or Event.

See the Transition Lifecycle for specifics on execution order and precedence between callbacks and actions.

import statesman


class InheritableStateMachine(statesman.StateMachine):
    async def guard_transition(self, transition: statesman.Transition, *args, **kwargs) -> bool:
        return True

    async def before_transition(self, transition: statesman.Transition, *args, **kwargs) -> None:
        ...

    async def on_transition(self, transition: statesman.Transition, *args, **kwargs) -> None:
        ...

    async def after_transition(self, transition: statesman.Transition, *args, **kwargs) -> None:
        ...

State machines can be asynchronously constructed and initialized:

import statesman


class States(statesman.StateEnum):
    starting = 'Starting'
    running = 'Running'
    stopping = 'Stopping'
    stopped = 'Stopped'


async def _example() -> None:
    state_machine = await statesman.StateMachine.create(
        states=statesman.State.from_enum(States)
    )
    print(f"Initialized state machine: {repr(state_machine)}")

The statesman.StateMachine.enter_state method provides great flexibility for putting the state machine into a specific state without regard for the event transition constraints that have been defined. Depending on the specifics of the state being modeled, this can become a potential liability as it can enable programmer errors by coercing an initialized state machine into an inconsistent or unexpected state. It can be desirable to restrict the use of enter_state to establishing an initial state or forbidding its use entirely in favor of initial state transition events.

As such, statesman provides a set of behaviors that govern how the enter_state method behaves. The behavior is configured via the state_entry attribute of the Config class nested within the state machine class.

There are four behaviors available for configuration via state_entry:

• statesman.Entry.allow (Default): The enter_state method can be called at any time.
• statesman.Entry.initial: The enter_state method can be called to establish an initial state and thereafter is forbidden.
• statesman.Entry.ignore: The enter_state method can never succeed and will fail and return silently when called.
• statesman.Entry.forbid: The enter_state method can never succeed and will raise an exception when called.

import statesman


class StateMachine(statesman.StateMachine):
    class States(statesman.StateEnum):
        starting = 'Starting...'
        running = 'Running...'
        stopping = 'Stopping...'
        stopped = statesman.InitialState('Terminated.')

    class Config:
        state_entry = statesman.Entry.forbid

It can be interesting to maintain a history of transitions that have occurred within a state machine. Statesman provides out of the box support for history tracking via the statesman.HistoryMixin class:

import statesman


class StateMachine(statesman.HistoryMixin, statesman.StateMachine):
    class States(statesman.StateEnum):
        ready = statesman.InitialState("Ready")
        analyzing = "Analyzing"

        awaiting_description = "Awaiting Description"
        awaiting_measurement = "Awaiting Measurement"
        awaiting_adjustment = "Awaiting Adjustment"

        done = "Done"
        failed = "Failed"


async def _example() -> None:
    state_machine = StateMachine()
    await state_machine.enter_state(StateMachine.States.analyzing)
    await state_machine.enter_state(StateMachine.States.awaiting_measurement)
    await state_machine.enter_state(StateMachine.States.done)
    await state_machine.enter_state(StateMachine.States.failed)

    print(f"The history is: {state_machine.history}")

There are use cases such as testing and protocol development in which it can become desirable for the state machine to follow a defined linear sequence of transitions.

Statesman supports such use cases via the statesman.SequencingMixin class. Transitions can be sequenced by passing coroutine objects into the sequence method. Coroutine objects are returned when you call an async function but do not await its execution. The coroutine objects freeze your desired state transition, which are queued for iterative execution by invoking the next_transition method.

from typing import List
import statesman


class StateMachine(statesman.SequencingMixin, statesman.StateMachine):
    class States(statesman.StateEnum):
        ready = statesman.InitialState("Ready")
        analyzing = "Analyzing"

        awaiting_description = "Awaiting Description"
        awaiting_measurement = "Awaiting Measurement"
        awaiting_adjustment = "Awaiting Adjustment"

        done = "Done"
        failed = "Failed"

    @statesman.event([States.ready, States.analyzing], States.awaiting_description)
    async def request_description(self) -> None:
        """Request a Description of application state from the servo."""
        ...

    @statesman.event([States.ready, States.analyzing], States.awaiting_measurement)
    async def request_measurement(self, metrics: List[str]) -> None:
        """Request a Measurement from the servo."""
        ...

    @statesman.event([States.ready, States.analyzing], States.awaiting_adjustment)
    async def recommend_adjustments(self, adjustments: List[str]) -> None:
        """Recommend Adjustments to the Servo."""
        ...


async def _example() -> None:
    state_machine = StateMachine()
    state_machine.sequence(
        state_machine.request_description(),
        state_machine.request_measurement(metrics=[...]),
        state_machine.recommend_adjustments([...]),
        state_machine.request_measurement(metrics=[...]),
        state_machine.recommend_adjustments([...]),
    )

    while True:
        transition = state_machine.next_transition()
        print(f"Executed transition: {repr(transition)}")
        if not transition:
            break

There is active work underway exploring alternate syntaxes for working with statesman via a state table syntax/interface.

It would also be interesting to follow in the tradition of other FSM libraries that have come before and support graphing/visualizations of statesman machines.

It seems inevitable that hierarchical state machine support will eventually land.

Klemen Verdnik contributed the statesman logo.

statesman is licensed under the terms of the Apache 2.0 license.