“如果 x"和“如果 x"之间的区别和“如果 x 不是无" [英] Difference between "if x" and "if x is not None"

问题描述

似乎if x"几乎是较长的if x is not None"语法的简写.它们在功能上是相同的，还是在某些情况下，对于给定的 x 值，两者的评估方式不同?

It appears that "if x" is almost like short-hand for the longer "if x is not None" syntax. Are they functionally identical or are there cases where for a given value of x the two would evaluate differently?

我认为在 Python 实现中的行为也应该是相同的 - 但如果有细微的差异，那就太好了.

I would assume the behavior should also be identical across Python implementations - but if there are subtle differences it would be great to know.

推荐答案

以下情况:

test = False 
test = "" 
test = 0
test = 0.0 
test = []
test = () 
test = {} 
test = set()

if 测试会有所不同:

if test: #False

if test is not None: #True 

之所以如此，是因为 is 测试身份，意义

This is the case because is tests for identity, meaning

test is not None

等价于

id(test) == id(None) #False

因此

(test is not None) is (id(test) != id(None)) #True

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