Python 'If not' syntax

Question

I'm a bit confused about how/why so many python developers use if not in their conditional statements.

for example, lets say we had a function,

def foo(bar = None):
    if not bar:
        bar = 2

But why go about this way? I mean, wouldn't doing if bar != None or if bar is not Nonebe more explicit? What does if not try to say?

This question has been asked before look here: http://stackoverflow.com/questions/100732/why-is-if-not-someobj-better-than-if-someobj-none-in-python — jlars62, May 24 '13 at 16:27
Because 'if not' checks for truth and '!=' checks for equality. — ecline6, May 24 '13 at 16:27

score 278 · Accepted Answer · answered May 24 '13 at 16:29

278

Yes, if bar is not None is more explicit, and thus better, assuming it is indeed what you want. That's not always the case, there are subtle differences: if not bar: will execute if bar is any kind of zero or empty container, or False. Many people do use not bar where they really do mean bar is not None.

answered May 24 '13 at 16:29

1

So if bar is `[]`, then it will execute? – May 24 '13 at 16:31
3

@EdgarAroutiounian Yes. Same or empty sets, tuples, dictionaries, strings, etc. – May 24 '13 at 16:32
10

Wow that's really confusing – Joe Phillips Aug 07 '18 at 16:41
24

This is the best possible answer, bar none. – easeout Mar 07 '20 at 22:50
Better Answer: https://stackoverflow.com/a/34376465/10730632 – Purushothaman Srikanth Feb 06 '21 at 16:57
@easeout :D :D :D – skeetastax Aug 18 '23 at 00:45

Python 'If not' syntax

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