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

Question

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.

Answer 1

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

Answer 2

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.

Answer 3

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

Answer 4

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

Answer 5

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