为什么我会在没有__contains__的情况下陷入无限循环?

问题描述

以下代码陷入无限循环:

The following code runs into an infinite loop:

class SubCommandMap:

    def __init__(self):
        self._command = dict()

    def __getitem__(self, key):
        return self._command.get(key)

    def __setitem__(self, key, value):
        self._command[key] = value

m = SubCommandMap()
" " in m   # <- why is this an infinite loop?

当然这是一个错误.假定 m 是不同类型的不同对象.但是，为什么这会导致无限循环而不是引发异常?

Of course this is an error. m was suppossed to be a different object of a different type.But why does this end up in an infinite loop instead of throwing an exception?

我添加了以下方法:

    def __contains__(self, other):
        raise NotImplementedError()

现在我收到一条适当的错误消息.

Now I am getting an appropriate error message.

在其他类似情况下，我需要注意避免无限循环吗?

Are there other, similar cases where I need to be careful to avoid an infinite loop?

推荐答案

在缺少 __ contains __ 的情况下， in 运算符将使用 __ getitem __ 您定义的方法来查看它是否获取您要查找的对象.它将依次将每个int传递给 __ getitem __ ，直到找到该项或该方法引发异常.如果这些都不发生，那么您将陷入无限循环.

In the absence of __contains__ the in operator will use the __getitem__ method you have defined to see if it gets the object you're looking for. It will pass every int in turn to __getitem__ until it finds the item or the method raises an exception. If neither of those happens, you have an infinite loop.

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