"if x" vs "if x == True" vs "if x is True"

I have a Python 2.7 program running a series of tools depending on what is requested 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:

x = True
x = 123
x = 'False'

On the other hand, the function doesn't run with:

x = False

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.

Solution

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: '' and u''
empty containers (such as lists, tuples and dictionaries)
anything that implements __bool__ (in Python3) to return False, or __nonzero__ (in Python2) to return False or 0.
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.