Apologies if this has been asked before, but I have searched in vain for an answer to my exact question. Basically, with Python 2.7, I have a program running a series of geoprocessing tools, depended on what is reqested via a series of True/False variables that the user adjusts in the script e.g.
x = True if x: run function
However, I have now discovered that x does not need to be literally “True” for the function to run. For example:
In: x = True if x: print True Out: True In: x = 123 if x: print True Out: True In: x = 'False' if x: print True Out: True In: x = False if x: print True Out:
So any value other than False appears to evaluate to True, which would not be the case for if x == True or if x is True. Seeing as PEP 8 strongly recommends only using the if x: variant, can anybody explain why this behaviour occurs? It seems that if x: is more a test for “if x is not False” or “if x exists”. With that in mind, I believe I should be using if x is True: in this case, despite what PEP 8 has to say.
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Answer
The following values in Python are false in the context of if
and other logical contexts:
False
None
- numeric values equal to 0, such as
0
,0.0
,-0.0
- empty strings:
''
andu''
- empty containers (such as lists, tuples and dictionaries)
- anything that implements
__bool__
(in Python3) to returnFalse
, or__nonzero__
(in Python2) to returnFalse
or0
. - anything that doesn’t implement
__bool__
(in Python3) or__nonzero__
(in Python2), but does implement__len__
to return a value equal to 0
An object is considered “false” if any of those applies, and “true” otherwise, regardless of whether it’s actually equal to or identical with False
or True
Now, if you’ve arranged that x
is necessarily one of the objects True
or False
, then you can safely write if x
. If you’ve arranged that the “trueness” of x
indicates whether or not to perform the operation, regardless of type, then you can safely write if x
. Where you can write that you should prefer to do so, since it’s cleaner to read.
Normally, if it is allowed for x
to take the value True
then you’re in one of those two cases, and so you would not write if x is True
. The important thing is to correctly document the meaning of x
, so that it reflects the test used in the code.
Python programmers are expected to know what’s considered true, so if you just document, “runs the function if x
is true”, then that expresses what your original code does. Documenting it, “runs the function if x is True
” would have a different meaning, and is less commonly used precisely because of the style rule in PEP8 that says to test for trueness rather than the specific value True
.
However, if you wanted the code to behave differently in the case where x
is an empty container from the case where it is None
, then you would write something like if x is not None
.