Python `if x is not None` or `if not x is None`?

Question

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. Is there any minor performance difference (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.

Answer 1

You should use if x is not None for readability sake because the bytecode compiler parses both if x is not None and if not x is None into x is not None.

if x is not None: 

X =5 #x can be anything depending on your need