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