为什么这不能作为数组成员测试?

问题描述

>>> 5 in [1, 2, 3, 4] == False
False

I get that this is a bizarre way to test membership, and that

>>> 5 not in [1, 2, 3, 4]
True

is the "correct" way. What confuses me is that its behavior is different from both

>>> (5 in [1, 2, 3, 4]) == False
True

and

>>> 5 in ([1, 2, 3, 4] == False)
TypeError ...

Have I missed something obvious? (Tested in Python 2.7 and Python 3.4).

To clarify, I understand the last three snippets. I am asking about the behavior of the first snippet, and why it is different.

解决方案

This is a chained comparison. You may have known that you can do

1 < 2 < 3

in Python, and it's equivalent to (1 < 2) and (2 < 3). (Or maybe you didn't. Now you know.) Well, the same thing applies to in and ==.

5 in [1, 2, 3, 4] == False

is equivalent to

(5 in [1, 2, 3, 4]) and ([1, 2, 3, 4] == False)

Since [1, 2, 3, 4] is not equal to False, the whole expression evaluates to False.