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, eg 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.

Answer 3

The first is what you want. It checks that both variables are False . The second only checks that var1 and var2 is False , which means that they are not both True simultaneously; one could be True and the other False , and the second if would return True .

Answer 4

Your first example is equivalent to

if not any((var1, var2)): ...

and your second is equivalent to

if not all((var1, var2)): ...

Those variants might represent the underlying problem better.

If they do, use those.

Answer 5

You should use the first example, it's more Pythonic.

Also, your second example is flawed. It will fail if 1 of your variables are True, and the other False.