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策略

策略在 Python 中的实现

策略是一种行为设计模式 它将一组行为转换为对象 并使其在原始上下文对象内部能够相互替换

原始对象被称为上下文 它包含指向策略对象的引用并将执行行为的任务分派给策略对象 为了改变上下文完成其工作的方式 其他对象可以使用另一个对象来替换当前链接的策略对象

进一步了解策略模式

在 Python 中使用模式

复杂度

流行度

使用示例 策略模式在 Python 代码中很常见 它经常在各种框架中使用 能在不扩展类的情况下向用户提供改变其行为的方式

识别方法 策略模式可以通过允许嵌套对象完成实际工作的方法以及允许将该对象替换为不同对象的设置器来识别

概念示例

本例说明了策略设计模式的结构并重点回答了下面的问题

  • 它由哪些类组成
  • 这些类扮演了哪些角色
  • 模式中的各个元素会以何种方式相互关联

main.py: 概念示例

from __future__ import annotations
from abc import ABC, abstractmethod
from typing import List


class Context():
    """
    The Context defines the interface of interest to clients.
    """

    def __init__(self, strategy: Strategy) -> None:
        """
        Usually, the Context accepts a strategy through the constructor, but
        also provides a setter to change it at runtime.
        """

        self._strategy = strategy

    @property
    def strategy(self) -> Strategy:
        """
        The Context maintains a reference to one of the Strategy objects. The
        Context does not know the concrete class of a strategy. It should work
        with all strategies via the Strategy interface.
        """

        return self._strategy

    @strategy.setter
    def strategy(self, strategy: Strategy) -> None:
        """
        Usually, the Context allows replacing a Strategy object at runtime.
        """

        self._strategy = strategy

    def do_some_business_logic(self) -> None:
        """
        The Context delegates some work to the Strategy object instead of
        implementing multiple versions of the algorithm on its own.
        """

        # ...

        print("Context: Sorting data using the strategy (not sure how it'll do it)")
        result = self._strategy.do_algorithm(["a", "b", "c", "d", "e"])
        print(",".join(result))

        # ...


class Strategy(ABC):
    """
    The Strategy interface declares operations common to all supported versions
    of some algorithm.

    The Context uses this interface to call the algorithm defined by Concrete
    Strategies.
    """

    @abstractmethod
    def do_algorithm(self, data: List):
        pass


"""
Concrete Strategies implement the algorithm while following the base Strategy
interface. The interface makes them interchangeable in the Context.
"""


class ConcreteStrategyA(Strategy):
    def do_algorithm(self, data: List) -> List:
        return sorted(data)


class ConcreteStrategyB(Strategy):
    def do_algorithm(self, data: List) -> List:
        return reversed(sorted(data))


if __name__ == "__main__":
    # The client code picks a concrete strategy and passes it to the context.
    # The client should be aware of the differences between strategies in order
    # to make the right choice.

    context = Context(ConcreteStrategyA())
    print("Client: Strategy is set to normal sorting.")
    context.do_some_business_logic()
    print()

    print("Client: Strategy is set to reverse sorting.")
    context.strategy = ConcreteStrategyB()
    context.do_some_business_logic()

Output.txt: 执行结果

Client: Strategy is set to normal sorting.
Context: Sorting data using the strategy (not sure how it'll do it)
a,b,c,d,e

Client: Strategy is set to reverse sorting.
Context: Sorting data using the strategy (not sure how it'll do it)
e,d,c,b,a

策略在其他编程语言中的实现

策略在 Java 中的实现 策略在 C# 中的实现 策略在 PHP 中的实现 策略在 Ruby 中的实现 策略在 Swift 中的实现 策略在 TypeScript 中的实现