Python What is more efficient if not == or if !=?

Question:

What is the difference between the following lines:

if not x == 'val':

Y

if x != 'val':

Is there a way to check if one is more efficient than the other?

or, maybe the following block would be better:

if x == 'val':
    pass
else:

Answer:

disassembled

If we use the disassembler for Python ( dis ) , we see that all 3 compile to the same number of instructions, and only differ in the POP_JUMP_IF_TRUE or POP_JUMP_IF_FALSE

!=
              0 LOAD_FAST                0 (x)
              3 LOAD_CONST               1 ('val')
              6 COMPARE_OP               3 (!=)
              9 POP_JUMP_IF_FALSE       15

---

not ==
              0 LOAD_FAST                0 (x)
              3 LOAD_CONST               1 ('val')
              6 COMPARE_OP               2 (==)
              9 POP_JUMP_IF_TRUE        15

---

== else
              0 LOAD_FAST                0 (x)
              3 LOAD_CONST               1 ('val')
              6 COMPARE_OP               2 (==)
              9 POP_JUMP_IF_FALSE       15

Demo in ideone

---

profile

Using cProfile , we can measure the execution time and perform a benchmark :

          ncalls  tottime  percall  cumtime  percall filename:lineno(function)
not ==   5000000    1.589    0.000    1.589    0.000 ./prog.py:8(en)
== else  5000000    1.562    0.000    1.562    0.000 ./prog.py:12(ee)
!=       5000000    1.508    0.000    1.508    0.000 ./prog.py:4(ne)

Sorted from slowest to fastest, we can see that:

• if not x == 'val' is the slowest.
• if x == 'val' + else is ~ 1.7% faster.
• if x != 'val' is ~ 3.6% even faster.
• Between the slowest and the fastest we only save 81ms / 5 million comparisons.

Demo in ideone

_{* Or here is a more complex example to compare true and false cases with random strings.}

---

conclusion

The if x != 'val' condition is slightly more efficient.

However, in my opinion, the difference is not significant enough to change the style when coding. I absolutely prefer the code to be as easy to read as possible, and I would only think of taking this result into account in extremely intensive calculations that require saving every possible fraction of a second.