Effect of the position of 'not' in python statement on performance

Question

In the application I am currently developing in python3, I often use statements like

elem_in_list = elem in list

But sometimes I need to check whether the element is not in the list. Is there a performance differece between

elem_not_in_list = not elem in list

and

elem_not_in_list = elem not in list

or is it just the same? Is one of the notations preferable?

Answer 1

These expressions compile to the exact same bytecode, so they're exactly equally efficient. not in tends to be more readable, but it's a matter of opinion.

>>> import dis
>>> def f(x, y):
...     return not x in y
...
>>> def g(x, y):
...     return x not in y
...
>>> dis.dis(f)
  2           0 LOAD_FAST                0 (x)
              3 LOAD_FAST                1 (y)
              6 COMPARE_OP               7 (not in)
              9 RETURN_VALUE
>>> dis.dis(g)
  2           0 LOAD_FAST                0 (x)
              3 LOAD_FAST                1 (y)
              6 COMPARE_OP               7 (not in)
              9 RETURN_VALUE