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.

...to compare singletons like None. Use is or is not.

Answer 1

There's no performance difference, as they compile to the same bytecode:

>>> import dis
>>> dis.dis("not x is None")
  1           0 LOAD_NAME                0 (x)
              2 LOAD_CONST               0 (None)
              4 COMPARE_OP               9 (is not)
              6 RETURN_VALUE
>>> dis.dis("x is not None")
  1           0 LOAD_NAME                0 (x)
              2 LOAD_CONST               0 (None)
              4 COMPARE_OP               9 (is not)
              6 RETURN_VALUE

Stylistically, I try to avoid not x is y, a human reader might misunderstand it as (not x) is y. If I write x is not y then there is no ambiguity.