"x not in" vs. "not x in" [duplicate]

Question

I've noticed that both of these work the same:

if x not in list and if not x in list.

Is there some sort of difference between the two in certain cases? Is there a reason for having both, or is it just because it's more natural for some people to write one or the other?

Which one am I more likely to see in other people's code?

Answer 1

The two forms make identical bytecode, as you can clearly verify:

>>> import dis >>> dis.dis(compile('if x not in d: pass', '', 'exec'))   1           0 LOAD_NAME                0 (x)               3 LOAD_NAME                1 (d)               6 COMPARE_OP               7 (not in)               9 JUMP_IF_FALSE            4 (to 16)              12 POP_TOP                           13 JUMP_FORWARD             1 (to 17)         >>   16 POP_TOP                      >>   17 LOAD_CONST               0 (None)              20 RETURN_VALUE         >>> dis.dis(compile('if not x in d: pass', '', 'exec'))   1           0 LOAD_NAME                0 (x)               3 LOAD_NAME                1 (d)               6 COMPARE_OP               7 (not in)               9 JUMP_IF_FALSE            4 (to 16)              12 POP_TOP                           13 JUMP_FORWARD             1 (to 17)         >>   16 POP_TOP                      >>   17 LOAD_CONST               0 (None)              20 RETURN_VALUE         

so obviously they're semantically identical.

As a matter of style, PEP 8 does not mention the issue.

Personally, I strongly prefer the if x not in y form -- that makes it immediately clear that not in is a single operator, and "reads like English". if not x in y may mislead some readers into thinking it means if (not x) in y, reads a bit less like English, and has absolutely no compensating advantages.