AsyncMachine transitions fail when a list of `State` objects are passed in to the FSM.

[mcvady]

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Describe the bug

AsyncMachine transitions fail when a list of State objects are passed in to the FSM.

Minimal working example

The following example works as expected. First A is printed. Then the transition is awaited/occurs, then B is printed.

import asyncio

from transitions import State
from transitions.extensions.asyncio import AsyncMachine

class AsyncModel:
    """Async Model"""

STATES = [
    "A",
    "B",
]

TRANSITIONS = [{"trigger": "progress", "source": "A", "dest": "B"}]

async def main():
    model = AsyncModel()
    _ = AsyncMachine(model=model, states=STATES, transitions=TRANSITIONS, initial="A")

    print(model.state)
    await model.progress()
    print(model.state)

if __name__ == "__main__":
    asyncio.get_event_loop().run_until_complete(main())

If STATES is defined as a list of dictionaries, the example still works fine. e.g:

STATES = [
    {"name": "A"},
    {"name": "B"},
]

However when STATES is a list of State objects, e.g:

STATES = [
    State(name="A"),
    State(name="B"),
]

... now awaiting the transition coroutine blows up:

A
/home/mcvady/tmp/.venv/lib/python3.10/site-packages/transitions/core.py:135: RuntimeWarning: coroutine 'AsyncMachine.callbacks' was never awaited
  event_data.machine.callbacks(self.on_exit, event_data)
RuntimeWarning: Enable tracemalloc to get the object allocation traceback
Traceback (most recent call last):
  File "/home/mcvady/tmp/async-example.py", line 25, in <module>
    asyncio.get_event_loop().run_until_complete(main())
  File "/usr/lib/python3.10/asyncio/base_events.py", line 649, in run_until_complete
    return future.result()
  File "/home/mcvady/tmp/async-example.py", line 21, in main
    await model.progress()
  File "/home/mcvady/tmp/.venv/lib/python3.10/site-packages/transitions/extensions/asyncio.py", line 177, in trigger
    return await self.machine.process_context(func, model)
  File "/home/mcvady/tmp/.venv/lib/python3.10/site-packages/transitions/extensions/asyncio.py", line 392, in process_context
    res = await self._process_async(func, model)
  File "/home/mcvady/tmp/.venv/lib/python3.10/site-packages/transitions/extensions/asyncio.py", line 447, in _process_async
    return await trigger()
  File "/home/mcvady/tmp/.venv/lib/python3.10/site-packages/transitions/extensions/asyncio.py", line 183, in _trigger
    await self._process(event_data)
  File "/home/mcvady/tmp/.venv/lib/python3.10/site-packages/transitions/extensions/asyncio.py", line 201, in _process
    event_data.result = await trans.execute(event_data)
  File "/home/mcvady/tmp/.venv/lib/python3.10/site-packages/transitions/extensions/asyncio.py", line 126, in execute
    await self._change_state(event_data)
  File "/home/mcvady/tmp/.venv/lib/python3.10/site-packages/transitions/extensions/asyncio.py", line 138, in _change_state
    await event_data.machine.get_state(self.source).exit(event_data)
TypeError: object NoneType can't be used in 'await' expression

Expected behavior Passing in list of explicit State objects should work the same using dictionaries or strings.

Additional context Using Python 3.10.12.

[aleneum]

Hello @mcvady,

this is unfortunately a common pitfall. AsyncMachine requires AsyncState as a state class. Simple State instances do not feature async processing.

from transitions.extensions.asyncio import AsyncState

Each machine class has a state_cls class property that is used to create state instances. Similarly, event_cls and transition_cls are used for the creation of events and transitions.

[mcvady]

Ah, I see @aleneum. Thank you for your work and the quick reply.