What is more 'pythonic' for 'not' [duplicate]

Solution 1:

The second option is more Pythonic for two reasons:

• It is one operator, translating to one bytecode operand. The other line is really not (4 in a); two operators.

  As it happens, Python optimizes the latter case and translates not (x in y) into x not in y anyway, but that is an implementation detail of the CPython compiler.

• It is close to how you'd use the same logic in the English language.

Solution 2:

Most would agree that 4 not in a is more Pythonic.

Python was designed with the purpose of being easy to understand and intelligible, and 4 not in a sounds more like how you would say it in English - chances are you don't need to know Python to understand what that means!

Note that in terms of bytecode, the two will be identical in CPython (although not in is technically a single operator, not 4 in a is subject to optimization):

>>> import dis
>>> def test1(a, n):
        not n in a


>>> def test2(a, n):
        n not in a


>>> dis.dis(test1)
  2           0 LOAD_FAST                1 (n)
              3 LOAD_FAST                0 (a)
              6 COMPARE_OP               7 (not in)
              9 POP_TOP             
             10 LOAD_CONST               0 (None)
             13 RETURN_VALUE        
>>> dis.dis(test2)
  2           0 LOAD_FAST                1 (n)
              3 LOAD_FAST                0 (a)
              6 COMPARE_OP               7 (not in)
              9 POP_TOP             
             10 LOAD_CONST               0 (None)
             13 RETURN_VALUE