Python: "not" keyword placement

Question

Which is more preferred below? and why?

Does PEP8 have a say in this?

if retcode != 0 and not "ignore_warning" in cmd_out:

or

if retcode != 0 and "ignore_warning" not in cmd_out:

functionally, i believe they achieve the same

score 3 · Accepted Answer · edited Dec 05 '19 at 01:39

3

Use "is not" operator rather than "not ... is" . While both expressions are functionally identical, the former is more readable and preferred.

PEP8

edited Dec 05 '19 at 01:39

Marcin Orlowski

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answered Dec 05 '19 at 01:37

yyforbidden

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2

This has nothing to do with the `in` operator though. – Selcuk Dec 05 '19 at 01:38
Why it should be about `in`? – Marcin Orlowski Dec 05 '19 at 01:40
1

@MarcinOrlowski because the original question is asking about `in` – user1558604 Dec 05 '19 at 01:43
@MarcinOrlowski I believe the point in this is about to make it more readable when using "not", regardless if it is used with "in" or "is". I found this answer as well, which explains more than I thought. https://stackoverflow.com/questions/17659303/what-is-more-pythonic-for-not – yyforbidden Dec 05 '19 at 01:46
2

The information you quoted is specifically about `is not` and not about the placement of `not` in general. – Selcuk Dec 05 '19 at 02:01
We can agree that PEP-8 is talking about `... is not ...`/`not ... is...`. But, is there anything about the *reason* given to prefer `is not` that doesn't apply to `not in` as well? – chepner Dec 05 '19 at 03:45

score 3 · Answer 2 · answered Dec 05 '19 at 01:45

Looking at the disassembled bytecode both the expressions are the same. From readability point of view second option is better.

if a != 10 and not b in "test":

and

if a != 10 and b not in "test":

gives the same disassembled code

  2           0 LOAD_FAST                0 (a)
              2 LOAD_CONST               1 (10)
              4 COMPARE_OP               3 (!=)
              6 POP_JUMP_IF_FALSE       26
              8 LOAD_FAST                1 (b)
             10 LOAD_CONST               2 ('test')
             12 COMPARE_OP               7 (not in)
             14 POP_JUMP_IF_FALSE       26

Python: "not" keyword placement

2 Answers2