python - 如何为 `transitions` 状态机定义触发器枚举？

Question

作为 this answer 的不相关跟进，它使用以下工作代码:

from transitions import Machine
from transitions import EventData
from typing import Callable
from enum import Enum, auto


class Observer:

    def state_changed(self, event_data: EventData):
        print(f"state is now '{event_data.state.name}'")


class State(Enum):
    SOLID = auto()
    LIQUID = auto()
    GAS = auto()


class SubscribableMachine(Machine):

    transitions = [
        {'trigger': 'heat', 'source': State.SOLID, 'dest': State.LIQUID},
        {'trigger': 'heat', 'source': State.LIQUID, 'dest': State.GAS},
        {'trigger': 'cool', 'source': State.GAS, 'dest': State.LIQUID},
        {'trigger': 'cool', 'source': State.LIQUID, 'dest': State.SOLID}
    ]

    def __init__(self):
        super().__init__(states=State, transitions=self.transitions,
                         initial=State.SOLID, send_event=True)

    def subscribe(self, func: Callable, state: State):
        self.get_state(state).on_enter.append(func)

    def unsubscribe(self, func: Callable, state: State):
        self.get_state(state).on_enter.remove(func)


machine = SubscribableMachine()
observer = Observer()
machine.subscribe(observer.state_changed, State.LIQUID)
machine.heat()  # >>> state is now 'LIQUID'
machine.heat()
assert machine.state == State.GAS
machine.unsubscribe(observer.state_changed, State.LIQUID)
machine.cool()  # no output
assert machine.state == State.LIQUID

我也想为 Trigger 设置一个枚举，就像我为 State 设置一个枚举一样。

唉，当我尝试的时候

class Trigger(Enum):
    heat = auto()
    cool = auto()

和

transitions = [
    {'trigger': Trigger.heat, 'source': State.SOLID, 'dest': State.LIQUID},
    {'trigger': Trigger.heat, 'source': State.LIQUID, 'dest': State.GAS},
    {'trigger': Trigger.cool, 'source': State.GAS, 'dest': State.LIQUID},
    {'trigger': Trigger.cool, 'source': State.LIQUID, 'dest': State.SOLID}
]

def __init__(self):
    super().__init__(states=State, transitions=self.transitions,
                     initial=State.SOLID, send_event=True)

我明白了

Traceback (most recent call last):
  File "C:/code/EPMD/Kodex/Algorithms/src/python/epmd/ablation_points/queries/dfgfdsg.py", line 42, in <module>
    machine = SubscribableMachine()
  File "C:/code/EPMD/Kodex/Algorithms/src/python/epmd/ablation_points/queries/dfgfdsg.py", line 33, in __init__
    initial=State.SOLID, send_event=True)
  File "C:\Code\EPMD\Kodex\EPD_Prerequisite\python_3.7.6\Lib\site-packages\transitions\core.py", line 589, in __init__
    self.add_model(model)
  File "C:\Code\EPMD\Kodex\EPD_Prerequisite\python_3.7.6\Lib\site-packages\transitions\core.py", line 607, in add_model
    self._add_trigger_to_model(trigger, mod)
  File "C:\Code\EPMD\Kodex\EPD_Prerequisite\python_3.7.6\Lib\site-packages\transitions\core.py", line 813, in _add_trigger_to_model
    self._checked_assignment(model, trigger, partial(self.events[trigger].trigger, model))
  File "C:\Code\EPMD\Kodex\EPD_Prerequisite\python_3.7.6\Lib\site-packages\transitions\core.py", line 807, in _checked_assignment
    if hasattr(model, name):
TypeError: hasattr(): attribute name must be string

---

我可以使用 Enum 的 .name 解决它:

    transitions = [
        {'trigger': Trigger.heat.name, 'source': State.SOLID, 'dest': State.LIQUID},
        {'trigger': Trigger.heat.name, 'source': State.LIQUID, 'dest': State.GAS},
        {'trigger': Trigger.cool.name, 'source': State.GAS, 'dest': State.LIQUID},
        {'trigger': Trigger.cool.name, 'source': State.LIQUID, 'dest': State.SOLID}
    ]

但是 State 和 Trigger 之间的不对称让我很困扰。
难道我做错了什么？为什么枚举适用于 State 而不是 Trigger？

Answer 1

作为vish已经指出，没有办法直接使用枚举作为触发器。 Transitions 将触发器名称作为方法绑定(bind)到模型，因此要求触发器是字符串。但是，Machine 和所有子类在编写时都考虑到了继承。如果你想使用枚举而不是类和类属性，你可以派生一个合适的EnumTransitionMachine并得到一个统一的接口(interface):

from transitions import Machine
from enum import Enum, auto


class State(Enum):
    A = auto()
    B = auto()
    C = auto()


class Transitions(Enum):
    GO = auto()
    PROCEED = auto()


class EnumTransitionMachine(Machine):

    def add_transition(self, trigger, *args, **kwargs):
        super().add_transition(trigger.name.lower() if hasattr(trigger, 'name') else trigger, *args, **kwargs)


transitions = [[Transitions.GO, State.A, State.B], [Transitions.PROCEED, State.B, State.C]]
m = EnumTransitionMachine(states=State, transitions=transitions, initial=State.A)
m.go()
assert m.state == State.B
m.proceed()
assert m.is_C()

仅供引用:还有可能将 Enums 与字符串值一起使用:

class State(Enum):
    A = "A"
    B = "B"
    C = "C"


class Transitions(Enum):
    GO = "GO"
    PROCEED = "PROCEED"

# you could use the enum's value then instead:
    def add_transition(self, trigger, *args, **kwargs):
        super().add_transition(trigger.value.lower() if hasattr(trigger, 'value') else trigger, *args, **kwargs)