CODEWITHSUNDEEP
pythonboolean
Goyuix
Goyuix

Reputation: 24330

Difference between "if x" and "if x is not None"

It appears that if x is almost like short-hand for the longer if x is not None syntax. Are they functionally identical or are there cases where for a given value of x the two would evaluate differently?

I would assume the behavior should also be identical across Python implementations - but if there are subtle differences it would be great to know.

Upvotes: 57

Views: 27416

Answers (5)

Ant
Ant

Reputation: 5404

if x checks if x is considered as True.

In Python, everything has a boolean value (True/False).

Values that are considered as False:

  • False, None
  • 0, 0.0, 0j
  • [], (), {}
  • ''
  • Other instances that signal to Python that they are empty

Other values are considered as True. For example, [False], ('hello'), 'hello' are considered as True (because they are not empty).

When using if x is not None, you are checking if x is not None, but it can be False or other instances that are considered as False.

>>> x = None
>>> if not x:print x # bool(None) is False
None
>>> if x == None:print x
None
>>> x = False
>>> if not x:print x
False
>>> if x == None:print x

Finally, note that True and False are respectively equal to 1 and 0:

>>> True + 1
2
>>> False + 1
1
>>> range(1, 5)[False]
1

Upvotes: 5

systempuntoout
systempuntoout

Reputation: 74064

In the following cases: 

test = False 
test = "" 
test = 0
test = 0.0 
test = []
test = () 
test = {} 
test = set()

the if test will differ:

if test: #False

if test is not None: #True

This is the case because is tests for identity, meaning

test is not None

is equivalent to 

id(test) == id(None) #False

therefore

(test is not None) is (id(test) != id(None)) #True

Upvotes: 71

Koroviev
Koroviev

Reputation: 337

if x:
    # Evaluates for any defined non-False value of x
if not x:
    # Evaluates for any defined False value of x
if x is None:
    # Evaluates for any instances of None

None is its own type, which happens to be False. "if not x" evaluates if x = None, only because None is False.

There aren't any subtle differences that I know of but there are exact methods to test for use for positivity/negativity in exact situations. Mixing them can work in some situations, but can lead to problems if they're not understood.

if x is True:
    # Use for checking for literal instances of True
if x is False:
    # Use for checking for literal instances of False
if x is None:
    # Use for checking for literal instances of None
if x:
    # Use for checking for non-negative values
if not x:
    # Use for checking for negative values
    # 0, "", None, False, [], (), {} are negative, all others are True

Upvotes: 4

eduffy
eduffy

Reputation: 40224

x = 0
if x: ...  # False
if x is not None: ... # True

Upvotes: 6

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 798526

The former tests trueness, whereas the latter tests for identity with None. Lots of values are false, such as False, 0, '', and None, but only None is None.

Upvotes: 37

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