“x not in” vs. “not x in” [duplicate]

by

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.

More Related Contents:

  1. Python: Does Python have a ternary conditional operator?
  2. “is” operator behaves unexpectedly with integers
  3. Bitwise operation and usage
  4. When is “i += x” different from “i = i + x” in Python?
  5. The tilde operator in Python
  6. and / or operators return value [duplicate]
  7. Asterisk in function call [duplicate]
  8. What exactly does += do?
  9. proper name for python * operator?
  10. Python: defining my own operators?
  11. How can I obtain the element-wise logical NOT of a pandas Series?
  12. What does % do to strings in Python?
  13. Python != operation vs “is not”
  14. Why is ” > 0 True in Python 2?
  15. Override Python’s ‘in’ operator?
  16. Why are there no ++ and –​ operators in Python?
  17. What is the difference between i = i + 1 and i += 1 in a ‘for’ loop? [duplicate]
  18. What is the motivation for the “or” operator to not return a bool?
  19. What does “ mean in Python?
  20. Modulo operator in Python
  21. Overloading Addition, Subtraction, and Multiplication Operators
  22. Is the behaviour of Python’s list += iterable documented anywhere?
  23. Python augmenting multiple variables inline
  24. What do these operators mean (** , ^ , %, //)? [closed]
  25. What is the difference between ‘+=’ and ‘=+’? [duplicate]
  26. Is there a short-hand for nth root of x in Python?
  27. How does Python’s bitwise complement operator (~ tilde) work?
  28. How do I get a decimal value when using the division operator in Python?
  29. Under what circumstances are __rmul__ called?
  30. Why should I use operator.itemgetter(x) instead of [x]?

Leave a Comment