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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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