Test for object identity when checking for None in Python

Test for object identity when checking for None in Python

Consistently use "is" or "is not" to compare variables to None,
because doing so is preferrable, as per PEP 8
(http://www.python.org/dev/peps/pep-0008/#programming-recommendations):

> Comparisons to singletons like None should always be done with is or
> is not, never the equality operators.