Python是==运算符优先级 [英] Python is, == operator precedence

问题描述

在Python3中，

a = b = 3
a is None == b is None

返回False，但是

(a is None) == (b is None)

返回True.因此，我假设仅根据此示例，==优先于 is .

returns True. So I would assume based on this example alone, == has precedence over is.

但是，

a = b = None
a is None == b is None

返回True.还有

(a is None) == (b is None)

返回True.但是

a is (None == b) is None

返回False.在这种情况下，似乎 is 的优先级高于==.

returns False. In this case, it would seem as if is has precedence over ==.

再举一个例子，这个表达式并不意味着要做什么，但是请忍受.如果我说

To give another example, and this expression isn't meant to do anything, but bear with me please. If I say

None is None == None

它返回True.但是以下两个都返回False.

it returns True. But both of the following return False.

None is (None == None)
(None is None) == None

很明显，Python并没有以严格的优先级来评估它们，但是我对正在发生的事情感到困惑.如何使用2个不同的运算符来评估此表达式，但与任一顺序都不相同?

So clearly, Python isn't evaluating these with some strict precedence, but I'm confused what is going on. How is it evaluating this expression with 2 different operators, but differently from either order?

推荐答案

您在这里看到的是操作员链接，根本没有优先级！

What you see here is operator chaining and there is no precedence involved at all!

Python支持类似的表达式

Python supports expressions like

1 < a < 3

测试数字是否在1到3之间；它等于(1 < a) and (a < 3)，除了a仅被评估一次.

To test that a number is in between 1 and 3; it's equal to (1 < a) and (a < 3) except that a is only evaluated once.

不幸的是，这也意味着例如

Unfortunately that also means that e.g.

None is None == None

实际上是指

(None is None) and (None == None)

这当然是对的，而您开始时使用的更长的示例

which is of course True, and the longer example you started with

a = b = 3
a is None == b is None

表示

(a is None) and (None == b) and (b is None)

，如果a和b均为None，则只能为True.

which can only be True if both a and b are None.

文档此处，请参见有关链接的内容.

Documentation here, see the bit about chaining.

有时非常有用，但在您最不期望的时候也会弹出！

Very useful sometimes but it also pops up when you least expect it!

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