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?
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 
timeitand find out?not in; these little words are hard to search for.