策略模式

这种设计模式称为策略模式。它用于定义一系列算法，封装每个算法，并使它们可互换。策略设计模式允许算法独立于使用它的客户端。

例如，动物可以以许多不同的方式行走。步行可被视为由不同类型的动物实施的策略：

from types import MethodType

class Animal(object):
    
    def __init__(self, *args, **kwargs):
        self.name = kwargs.pop('name', None) or 'Animal'
        if kwargs.get('walk', None):
            self.walk = MethodType(kwargs.pop('walk'), self)

    def walk(self):
        """
        Cause animal instance to walk
        
        Walking funcionallity is a strategy, and is intended to
        be implemented separately by different types of animals.
        """
        message = '{} should implement a walk method'.format(
            self.__class__.__name__)
        raise NotImplementedError(message)

# Here are some different walking algorithms that can be used with Animal
def snake_walk(self):
    print('I am slithering side to side because I am a {}.'.format(self.name))

def four_legged_animal_walk(self):
    print('I am using all four of my legs to walk because I am a(n) {}.'.format(
        self.name))

def two_legged_animal_walk(self):
    print('I am standing up on my two legs to walk because I am a {}.'.format(
        self.name))

运行此示例将生成以下输出：

generic_animal = Animal()
king_cobra = Animal(name='King Cobra', walk=snake_walk)
elephant = Animal(name='Elephant', walk=four_legged_animal_walk)
kangaroo = Animal(name='Kangaroo', walk=two_legged_animal_walk)

kangaroo.walk()
elephant.walk()
king_cobra.walk()
# This one will Raise a NotImplementedError to let the programmer
# know that the walk method is intended to be used as a strategy.
generic_animal.walk()

    # OUTPUT:
    #
    # I am standing up on my two legs to walk because I am a Kangaroo.
    # I am using all four of my legs to walk because I am a(n) Elephant.
    # I am slithering side to side because I am a King Cobra.
    # Traceback (most recent call last):
    #   File "./strategy.py", line 56, in <module>
    #     generic_animal.walk()
    #   File "./strategy.py", line 30, in walk
    #     raise NotImplementedError(message)
    # NotImplementedError: Animal should implement a walk method 

请注意，在 C++或 Java 等语言中，此模式使用抽象类或接口来实现，以定义策略。在 Python 中，只需在外部定义一些可以使用 types.MethodType 动态添加到类中的函数更有意义。