在以下情况下：
test = False
test = ""
test = 0
test = 0.0
test = []
test = ()
test = {}
test = set()
该if测试将有所不同：
if test: #False
if test is not None: #True
之所以如此，是因为is测试身份，含义
test is not None
相当于
id(test) == id(None) #False
因此
(test is not None) is (id(test) != id(None)) #True
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