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.
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
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