Difference between if not: and if False:

Question

I have a probably quite simple question but was wondering between the difference of these two statements:

if not os.path.isfile(file):
  #Do some stuff

if os.path.isfile(file) is False:
  #Do some stuff

What are the differences (if any) between the two? To my understanding they both return a True or False value, so is it just a matter of preference or are there any significant differences?

Possible duplicate of [Understanding Python's "is" operator](https://stackoverflow.com/questions/13650293/understanding-pythons-is-operator) — Rach Sharp, May 22 '19 at 13:14
*wow*, did this really require 6 answers (plus some deleted) when there is a wealth of better information on SO e.g. https://stackoverflow.com/a/100762/4711754 — Andrew Allen, May 22 '19 at 13:23

score 2 · Accepted Answer · answered May 22 '19 at 13:18

In python (and other dynamic languages) there is the concept of truthy/falsy value. True/False are not the only things that evaluate as true/false

if not []: 
   print('this will be printed')

if [] is False:
   print('this won't)

Another problem is that you should compare with x == False, and not x is False. The False is a singleton object in the current implementation of CPython, but this is not guaranteed by the specification.

score 1 · Answer 2 · answered May 22 '19 at 13:15

In your case, since we know os.path.isfile returns True or False, there is no difference.

In general, there are a lot of objects in python which, when interpreted as boolean, will evaluate to False.

Think of this:

empty_list = []
if not empty_list:
    print('List is not empty')
if empty_list is False:
    print('List is False')

Among the others, None, "" and [] will evaluate to False.

So testing with not variable is usually the preferred way.

score 1 · Answer 3 · answered May 22 '19 at 13:27

It's usually better to use the first, since it works even if you're not checking an actual boolean value in a Python implementation where False is a singleton object.
Uniformity is good, and so is portability.

>>> if 0 is False: print "false"
>>> if not 0: print "false"
false
>>> if [] is False: print "false"
>>> if not []: print "false"
false
>>> if "" is False: print "false"
>>> if not "": print "false"
false

It also protects against mishaps like this:

>>> False = 1
>>> True == False
True

milanbalazs · Answer 4 · 2019-05-22T13:27:56.090

0

You should know: False == 0 == None in case of if condition. If you use if not, you can cover all version of False (zero value). If you use == False you cannot handle the 0 or None. if not is recommended. The is operator is a different story (is not same as ==) but you can read more details on this link: Understanding Python's "is" operator

edited May 22 '19 at 13:27

answered May 22 '19 at 13:11

milanbalazs

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Actually it does handle `0`, as it is treated as being falsey – yatu May 22 '19 at 13:12
2

False does not equal None – Alec May 22 '19 at 13:14
@yatu `False` and `0` are even more closely related than other falsey values, since `bool` is a subclass of `int`. – chepner May 22 '19 at 13:15
`None` is `NoneType` whether `False` is Boolean type. – Arkistarvh Kltzuonstev May 22 '19 at 13:15
I see your point, of course these values are not equal or same, but regarding to question in case of using `if not ...` condition doesn't matter that `0` or `False` or `None`. – milanbalazs May 22 '19 at 13:22
@milanbalazs you should edit your answer to indicate that False == 0 == None in the case of if conditions. – AlexeyGy May 22 '19 at 13:26
Thanks @ AlexeyGy. I have update. Probably it's more understandable what I meant in my answer. – milanbalazs May 22 '19 at 13:29

score 0 · Answer 5 · answered May 22 '19 at 13:16

0

False is just a keyword for the integer 0. Any "fasly" value will evaluate to False in an if not statement , but only 0 or False itself will evaluate to is False.

This is because is checks for matching identity, while if not checks for implicit booleanness.

For example:

if not None

evaluates.

if None is False

doesn't.

answered May 22 '19 at 13:16

Alec

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This is incorrect (in Python3): `0 is not False` – Jack Deeth Feb 07 '22 at 15:57

Difference between if not: and if False:

5 Answers5