为什么这不能用作数组成员资格测试? [英] Why does this not work as an array membership test?

问题描述

<预><代码>>>>5 in [1, 2, 3, 4] == False错误的

我知道这是一种测试会员资格的奇怪方式，而且

<预><代码>>>>5 不在 [1, 2, 3, 4] 中真的

是正确"的方式.让我困惑的是它的行为与两者不同

<预><代码>>>>([1, 2, 3, 4] 中的 5) == False真的

和

<预><代码>>>>5 in ([1, 2, 3, 4] == False)类型错误...

我是否遗漏了一些明显的东西?(在 Python 2.7 和 Python 3.4 中测试).

为了澄清，我理解最后三个片段.我在询问 first 代码段的行为，以及为什么不同.

解决方案

这是一个链式比较.你可能知道你可以做到

1 <2＜3

在 Python 中，它等价于 (1 <2) 和 (2 <3).(或许你没有.现在你知道了.)好吧，同样的事情适用于 in 和 ==.

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

相当于

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

由于 [1, 2, 3, 4] 不等于 False，整个表达式的计算结果为 False.

>>> 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.

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