使用关键字Args在Python中初始化，而无需在实例之间共享 [英] Initialising in Python using Keyword Args without sharing between instances

问题描述

如何在不使用类实例之间共享的情况下在初始化程序中使用关键字args?

How can I use keyword args in an initialiser without sharing it between instances of a class?

以下不良行为的示例，如果我要向集合foo中添加任何内容，则将在这两种情况下都将其添加.

Example of the bad behaviour below, if I were to add anything to the set foo then it would be added in both instances.

In [1]: class Klass:
    def __init__(self, foo=set()):
        self.foo = foo
In [2]: a = Klass()
In [3]: b = Klass()
In [4]: a.foo is b.foo
Out[4]: True

推荐答案

请注意，可变默认参数仅会出现此问题-请参见.要使用可变的默认参数，通常在函数定义中将默认值设置为None，然后在函数内部检查是否已提供值:

Note that this issue will only occur with mutable default arguments - see "Least Astonishment" and the Mutable Default Argument. To use a mutable default argument, it is conventional to set the default to None in the function definition, then check inside the function whether or not a value has been supplied:

class Klass:

    def __init__(self, foo=None):
        if foo is None:
            foo = set()
        self.foo = foo

或:

self.foo = foo if foo is not None else set()

请注意，None是通过身份(if foo is None:)进行测试的，而不是相等性(if foo == None:)或真实性(if not foo:)进行的测试.例如，如果您要显式传递已引用else的空集，则后者是一个问题.

Note that None is tested by identity (if foo is None:), not equality (if foo == None:) or truthiness (if not foo:). For example, the latter is an issue if you want to explicitly pass in an empty set you have referenced elsewere.

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