Python状态机模式

# Python 状态机模式 (State Machine)
# State 协议，FSM 类，状态转换，进入/退出动作，守卫条件
# 状态机将对象行为封装在不同状态中，状态变化时行为随之改变。

from abc import ABC, abstractmethod
from typing import Callable

# 状态接口
class State(ABC):
@abstractmethod
def on_enter(self, fsm: "OrderFSM") -> None: ...
@abstractmethod
def on_exit(self, fsm: "OrderFSM") -> None: ...
@abstractmethod
def handle(self, fsm: "OrderFSM", event: str) -> None: ...

# 具体状态
class PendingState(State):
def on_enter(self, fsm: "OrderFSM") -> None:
fsm.order.status = "pending"
def on_exit(self, fsm: "OrderFSM") -> None: pass
def handle(self, fsm: "OrderFSM", event: str) -> None:
if event == "pay":
fsm.transition_to(PaidState())
elif event == "cancel":
fsm.transition_to(TerminalState("cancelled"))

class PaidState(State):
def on_enter(self, fsm: "OrderFSM") -> None:
fsm.order.status = "paid"
def on_exit(self, fsm: "OrderFSM") -> None: pass
def handle(self, fsm: "OrderFSM", event: str) -> None:
if event == "ship":
fsm.transition_to(ShippedState())
elif event == "refund":
fsm.transition_to(TerminalState("refunded"))

class ShippedState(State):
def on_enter(self, fsm: "OrderFSM") -> None:
fsm.order.status = "shipped"
def on_exit(self, fsm: "OrderFSM") -> None: pass
def handle(self, fsm: "OrderFSM", event: str) -> None:
if event == "deliver":
fsm.transition_to(TerminalState("delivered"))

class TerminalState(State):
def __init__(self, status: str):
self._status = status
def on_enter(self, fsm: "OrderFSM") -> None:
fsm.order.status = self._status
def on_exit(self, fsm: "OrderFSM") -> None: pass
def handle(self, fsm: "OrderFSM", event: str) -> None: pass

# 守卫条件
class Guard:
def __init__(self, condition: Callable[[], bool]):
self._condition = condition
def passes(self) -> bool:
return self._condition()

# 有限状态机
class Order:
status: str = "pending"

class OrderFSM:
def __init__(self, order: Order):
self.order = order
self._current: State = PendingState()
self._guards: dict[tuple[type, str], Guard] = {}
def add_guard(self, st: type, evt: str, g: Guard) -> None:
self._guards[(st, evt)] = g
def transition_to(self, new: State) -> None:
self._current.on_exit(self)
self._current = new
self._current.on_enter(self)
def handle(self, event: str) -> None:
g = self._guards.get((type(self._current), event))
if g and not g.passes():
raise RuntimeError("守卫阻止了转换")
self._current.handle(self, event)

if __name__ == "__main__":
order = Order(); fsm = OrderFSM(order)
fsm.handle("pay"); print(f"状态: {order.status}")
fsm.handle("ship"); print(f"状态: {order.status}")
fsm.handle("deliver"); print(f"终态: {order.status}")