为什么'True == not False'是Python中的语法错误?

2022-01-19 00:00:00 python boolean comparison-operators

问题描述

在 Python 中使用 == 比较布尔值.但是当我应用布尔 not 运算符时，结果是语法错误:

Comparing boolean values with == works in Python. But when I apply the boolean not operator, the result is a syntax error:

Python 2.7 (r27:82500, Sep 16 2010, 18:02:00) 
[GCC 4.5.1 20100907 (Red Hat 4.5.1-3)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> True == True
True
>>> False == False
True
>>> True is not False
True
>>> True == not False
  File "<stdin>", line 1
    True == not False
              ^
SyntaxError: invalid syntax
>>>

为什么这是一个语法错误?我希望 not False 是一个返回布尔值的表达式，并且 True == <x> 在 <x><的任何地方都是有效的语法/code> 是具有有效语法的表达式.

Why is this a syntax error? I would expect not False to be an expression that returns a boolean value, and True == <x> to be valid syntax wherever <x> is an expression with valid syntax.

解决方案

和Python 中的运算符优先级(解释器认为您在比较 True 和 not，因为 == 的优先级高于 not).您需要一些括号来阐明操作顺序:

It has to do with operator precedence in Python (the interpreter thinks you're comparing True to not, since == has a higher precedence than not). You need some parentheses to clarify the order of operations:

True == (not False)

一般情况下，您不能在没有括号的比较右侧使用 not.但是，我想不出你需要在比较右侧使用 not 的情况.

In general, you can't use not on the right side of a comparison without parentheses. However, I can't think of a situation in which you'd ever need to use a not on the right side of a comparison.

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