python: if not this and not that

Question

I want to write a condition which checks if both variables are False but I'm not sure if what I'm using is correct:

if not var1 and not var2: 
     #do something 

or should it be:

if not (var1 and var2):
    #do something

Answer 1

This is called De Morgan's Law. (not A) and (not B) is equivalent to not (A or B), and (not A) or (not B) is equivalent to not (A and B).

Technically, it's very slightly faster to use the latter, e.g. not (A and B), as the interpreter evaluates fewer logical statements, but it's trivial. Use the logical structure that allows you to state the condition as clearly as possible, on a case-by-case basis.

Answer 2

if not (var1 and var2) is equivalent to if not var1 or not var2 so your conditions are not the same.

Which one is correct depends on your business logic.