说实话，这个问题我之前也不知道是为什么，于是抱着求知的心态，问了国外的大神，Михаил Никонов的回答原文如下：It's because of comparison operator chaining, I think. In Python, expression (x < y < z) is, unlike in C, equivalent to (x < y and y < z), and that can be used for any sequence of comparisons (including membership tests). So basically, without parentheses, your last expression is equivalent to ('a' in 'abc' and 'abc' == True), or so it seems.翻译过来差不多就是，print('a' in 'abc' == True) 这种表达式，在Python里面不像C语言那样，它必须要增加一个括号：print(('a' in 'abc') == True) 这样才对，如果不增加括号，就相当于print('a' in 'abc' and 'abc' == True) 结果当然是 False。