Python if not vs if

Question

What is the difference between these two lines of code:

if not x == 'val':
and

if x != 'val':
Is one more efficient than the other?

Answer 1

Using dis to look at the bytecode generated for the two versions:

not ==

  4           0 LOAD_FAST                0 (foo)
              3 LOAD_FAST                1 (bar)
              6 COMPARE_OP               2 (==)
              9 UNARY_NOT           
             10 RETURN_VALUE   
!=

  4           0 LOAD_FAST                0 (foo)
              3 LOAD_FAST                1 (bar)
              6 COMPARE_OP               3 (!=)
              9 RETURN_VALUE   
 

The latter has fewer operations, and is therefore likely to be slightly more efficient.