MRE

from transitions import Machine


class TradingSystem:
    def __init__(self):
        self.machine = Machine(model=self, states=['RUNNING'], initial='RUNNING')

    def check_running(self) -> None:
        if self.is_RUNNING():              
            print("System is running")

Example usage

system = TradingSystem()
system.check_running()

Issue

mypy transitions_mypy.py

gives the error:

transitions_mypy.py:9: error: "TradingSystem" has no attribute "is_RUNNING"  [attr-defined]

This can be avoided by bypassing mypy, for example adding # type: ignore[attr-defined] at the end of line 9.

But what is the proper way? Is it better to avoid bypassing mypy? Perhaps by manually defining the attribute?

The answer @Mark proposed won't work since transitions does not override already existing model attributes because that would be unexpected (mis)behaviour (as @Mark pointed out). If you enable logging, transitions will also tell you that:

import logging
logging.basicConfig(level=logging.DEBUG)

The output should contain:

WARNING:transitions.core:Model already contains an attribute 'is_RUNNING'. Skip binding.

In the past, I suggested to inherit from Machine and override Machine._checked_assignment to workaround that safeguard. However, @james-hirschorn's approach using attrs which was posted in the transitions issue tracker is a better solution in my oppinion:

from typing import Callable
from attrs import define, field
from transitions import Machine


@define(slots=False)
class TradingSystem:
    is_RUNNING: Callable[[], bool] = field(init=False)
    stop: Callable[[], None] = field(init=False)

    def __attrs_post_init__(self):
        self.machine = Machine(model=self, states=['RUNNING', 'STOPPED'], initial='RUNNING')
        self.machine.add_transition(trigger='stop', source='RUNNING', dest='STOPPED')

    def check_running(self) -> None:
        if self.is_RUNNING():  
            print("System is running")

    def stop_system(self) -> None:
        self.stop()
        print("System stopped")

# Example usage
system = TradingSystem()
system.check_running()
system.stop_system()

This produces some overhead since you have to define triggers and convenience methods twice. Furthermore, type information are also incomplete because transitions will also add is_STOPPED and auto transitions like to_STOPPED/RUNNING or 'peek' transition methods such as may_stop to TradingSystem during runtime.

Improving transitions typing support is currently an open issue.