Tim Martin Tim Martin - 4 months ago 16
Python Question

Why is 'True == not False' a syntax error in Python?

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
>>>


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.

Answer

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)

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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