Python责任链模式代码示例

该模式允许多个对象来对请求进行处理， 而无需让发送者类与具体接收者类相耦合。 链可在运行时由遵循标准处理者接口的任意处理者动态生成

责任链模式在 Python 程序中并不常见， 因为它仅在代码与对象链打交道时才能发挥作用

该模式可通过一组对象的行为方法间接调用其他对象的相同方法来识别， 而且所有对象都会遵循相同的接口

from __future__ import annotations
from abc import ABC, abstractmethod
from typing import Any, Optional


class Handler(ABC):
    """
    The Handler interface declares a method for building the chain of handlers.
    It also declares a method for executing a request.
    """

    @abstractmethod
    def set_next(self, handler: Handler) -> Handler:
        pass

    @abstractmethod
    def handle(self, request) -> Optional[str]:
        pass


class AbstractHandler(Handler):
    """
    The default chaining behavior can be implemented inside a base handler
    class.
    """

    _next_handler: Handler = None

    def set_next(self, handler: Handler) -> Handler:
        self._next_handler = handler
        # Returning a handler from here will let us link handlers in a
        # convenient way like this:
        # monkey.set_next(squirrel).set_next(dog)
        return handler

    @abstractmethod
    def handle(self, request: Any) -> str:
        if self._next_handler:
            return self._next_handler.handle(request)

        return None


"""
All Concrete Handlers either handle a request or pass it to the next handler in
the chain.
"""


class MonkeyHandler(AbstractHandler):
    def handle(self, request: Any) -> str:
        if request == "Banana":
            return f"Monkey: I'll eat the {request}"
        else:
            return super().handle(request)


class SquirrelHandler(AbstractHandler):
    def handle(self, request: Any) -> str:
        if request == "Nut":
            return f"Squirrel: I'll eat the {request}"
        else:
            return super().handle(request)


class DogHandler(AbstractHandler):
    def handle(self, request: Any) -> str:
        if request == "MeatBall":
            return f"Dog: I'll eat the {request}"
        else:
            return super().handle(request)


def client_code(handler: Handler) -> None:
    """
    The client code is usually suited to work with a single handler. In most
    cases, it is not even aware that the handler is part of a chain.
    """

    for food in ["Nut", "Banana", "Cup of coffee"]:
        print(f"\nClient: Who wants a {food}?")
        result = handler.handle(food)
        if result:
            print(f"  {result}", end="")
        else:
            print(f"  {food} was left untouched.", end="")


if __name__ == "__main__":
    monkey = MonkeyHandler()
    squirrel = SquirrelHandler()
    dog = DogHandler()

    monkey.set_next(squirrel).set_next(dog)

    # The client should be able to send a request to any handler, not just the
    # first one in the chain.
    print("Chain: Monkey > Squirrel > Dog")
    client_code(monkey)
    print("\n")

    print("Subchain: Squirrel > Dog")
    client_code(squirrel)