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

Question

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 1

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.