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

Posted on

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
>>> False == False
>>> True is not False
>>> 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.

Answered By: Rafe Kettler

Answer #2:

It’s just a matter of operator precedence. Try:

>>> True == (not False)

Have a look in this table of operator precedences, you’ll find that == binds tigher than not, and thus True == not False is parsed as (True == not) False which is clearly an error.

Answered By: unwind

Answer #3:

I think what you are looking for is “and not”. This gives you the results you are looking towards. If your comparing booleans what you have is a compound boolean expression, here is an example website Compound Boolean Expression.

>>> True and True
>>> True and not True
>>> True and not False
>>> False and not True
>>> False and not False
>>> False and False
Answered By: Nick

Leave a Reply

Your email address will not be published. Required fields are marked *