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.
Kind regards
The following values in Python are false in the context of if
and other logical contexts:
False
None
0
, 0.0
, -0.0
''
and u''
__bool__
(in Python3) to return False
, or __nonzero__
(in Python2) to return False
or 0
.__bool__
(in Python3) or __nonzero__
(in Python2), but does implement __len__
to return a value equal to 0An 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
.
Thank you for explaining in detail. In my case, while I only would want the user to use True or False, and no other values, the code is documented enough to follow PEP 8 and leave my if x: statements as they are.