Understanding precedence of `not` operator

Question

I tried to test empty string in Python (empty string should be "Falsy" as they write in: How to check if the string is empty?). However I used a little bit different syntax and get weird result in the following comparison:

not(''); # result: True

not('hello'); # result: False

not('hello') == False; # result: True

not('hello') == True; # result: True - how is this result possible? (False == True: result must be False)

Thank you for the answer!

`not` negates a boolean value. Empty strings are Falsy. not Falsy is True. - Non-empty strings are Thruthy, not Truthy is flase. `not` is no function - the ( ) is your doing. `not (hello) == True` is equal to `not ( "hello" == True)` — Patrick Artner, Oct 26 '20 at 10:42

score 2 · Answer 1 · answered Oct 26 '20 at 10:41

The precedence here is not ('hello' == False). 'hello' equals neither True nor False, so both 'hello' == True and 'hello' == False are False, which is then negated by not.

>>> 'hello' == False
False
>>> 'hello' == True
False
>>> not ('hello' == True)
True

Truthiness isn't the same as being equal to True. A string can be truthy (i.e. you can decide whether it's more of a "yes" or a "no"), but at the same time not be equal to a boolean value (because a string is a string and a boolean is a boolean).

score 2 · Answer 2 · answered Oct 26 '20 at 10:42

It's important to understand that not is an operator, not a function. The parenthesis do nothing for the expression, here's how it is read:

not('hello') == True
# is the same as
not 'hello' == True
# which is the same as
not ('hello' == True)
# which is equivalent to 
not False
# which is 
True

Which happens to evaluate to the same as the expression above (for the same reason that 'hello' == False is False.

The correct way of enforcing precedence with not would be to do

(not something) == True

Understanding precedence of `not` operator

2 Answers2