Python

Usage examples: The Chain of Responsibility is pretty common in Python. It’s mostly relevant when your code operates with chains of objects, such as filters, event chains, etc.

Identification: The pattern is recognizable by behavioral methods of one group of objects that indirectly call the same methods in other objects, while all the objects follow the common interface.

Conceptual Example

This example illustrates the structure of the Chain of Responsibility design pattern. It focuses on answering these questions:

• What classes does it consist of?

• What roles do these classes play?

• In what way the elements of the pattern are related?

main.py: Conceptual example

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)

Output.txt: Execution result

Chain: Monkey > Squirrel > Dog

Client: Who wants a Nut?
  Squirrel: I'll eat the Nut
Client: Who wants a Banana?
  Monkey: I'll eat the Banana
Client: Who wants a Cup of coffee?
  Cup of coffee was left untouched.

Subchain: Squirrel > Dog

Client: Who wants a Nut?
  Squirrel: I'll eat the Nut
Client: Who wants a Banana?
  Banana was left untouched.
Client: Who wants a Cup of coffee?
  Cup of coffee was left untouched.