如何使用静态方法作为策略设计模式的默认参数？ [英] How can I use a static method as a default parameter for the strategy design pattern?

问题描述

我想创建一个使用类似于以下策略设计模式的类：

I want to make a class that uses a strategy design pattern similar to this:

class C:

    @staticmethod
    def default_concrete_strategy():
        print("default")

    @staticmethod
    def other_concrete_strategy():
        print("other")

    def __init__(self, strategy=C.default_concrete_strategy):
        self.strategy = strategy

    def execute(self):
        self.strategy()

这会产生错误：

NameError: name 'C' is not defined

将 strategy = C.default_concrete_strategy 替换为 strategy = default_concrete_strategy 即可，但是默认情况下，策略实例变量将是静态方法对象而不是可调用方法。

Replacing strategy=C.default_concrete_strategy with strategy=default_concrete_strategy will work but, left as default, the strategy instance variable will be a static method object rather than a callable method.

TypeError: 'staticmethod' object is not callable

如果删除 @staticmethod，它将起作用装饰器，但是还有其他方法吗？我希望默认参数可以自我记录，以便其他人可以立即看到如何包含策略的示例。

It will work if I remove the @staticmethod decorator, but is there some other way? I want the default parameter to be self documented so that others will immediately see an example of how to include a strategy.

此外，有没有更好的方法来公开策略而不是而不是静态方法？我认为在这里实现完整的类没有意义。

Also, is there a better way to expose strategies rather than as static methods? I don't think that implementing full classes makes sense here.

推荐答案

不，您不能，因为 class 定义尚未完成运行，因此该类名称在当前名称空间中尚不存在。

No, you cannot, because the class definition has not yet completed running so the class name doesn't exist yet in the current namespace.

您可以直接使用函数对象：

class C:    
    @staticmethod
    def default_concrete_strategy():
        print("default")

    @staticmethod
    def other_concrete_strategy():
        print("other")

    def __init__(self, strategy=default_concrete_strategy.__func__):
        self.strategy = strategy

C 在定义方法时尚不存在，因此您使用本地名称引用 default_concrete_strategy 。 .__ func __ 解开 staticmethod 描述符以访问基础原始函数（ staticmethod 描述符本身不是可调用的。）

C doesn't exist yet when the methods are being defined, so you refer to default_concrete_strategy by the local name. .__func__ unwraps the staticmethod descriptor to access the underlying original function (a staticmethod descriptor is not itself callable).

另一种方法是使用前哨默认值。 None 在这里可以正常工作，因为 strategy 的所有正常值都是静态函数：

Another approach would be to use a sentinel default; None would work fine here since all normal values for strategy are static functions:

class C:    
    @staticmethod
    def default_concrete_strategy():
        print("default")

    @staticmethod
    def other_concrete_strategy():
        print("other")

    def __init__(self, strategy=None):
        if strategy is None:
            strategy = self.default_concrete_strategy
        self.strategy = strategy

自此检索 default_concrete_strategy 来自 self 调用描述符协议，并且（$ un $）函数由 static方法返回描述符本身，在类定义完成之后。

Since this retrieves default_concrete_strategy from self the descriptor protocol is invoked and the (unbound) function is returned by the staticmethod descriptor itself, well after the class definition has completed.

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