I've always thought of the if not x is None version to be more clear, but Google's style guide and PEP-8 both use if x is not None. Are there any minor performance differences (I'm assuming not), and is there any case where one really doesn't fit (making the other a clear winner for my convention)?*
*I'm referring to any singleton, rather than just None.
...to compare singletons like None. Use is or is not.