[python] Python if not == vs if !=

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?

Would it be better to use

if x == 'val':
    pass
else:

In the first one Python has to execute one more operations than necessary(instead of just checking not equal to it has to check if it is not true that it is equal, thus one more operation). It would be impossible to tell the difference from one execution, but if run many times, the second would be more efficient. Overall I would use the second one, but mathematically they are the same

It's about your way of reading it. not operator is dynamic, that's why you are able to apply it in

if not x == 'val':

But != could be read in a better context as an operator which does the opposite of what == does.

@jonrsharpe has an excellent explanation of what's going on. I thought I'd just show the difference in time when running each of the 3 options 10,000,000 times (enough for a slight difference to show).

Code used:

def a(x):
    if x != 'val':
        pass


def b(x):
    if not x == 'val':
        pass


def c(x):
    if x == 'val':
        pass
    else:
        pass


x = 1
for i in range(10000000):
    a(x)
    b(x)
    c(x)

And the cProfile profiler results:

So we can see that there is a very minute difference of ~0.7% between if not x == 'val': and if x != 'val':. Of these, if x != 'val': is the fastest.

However, most surprisingly, we can see that

if x == 'val':
        pass
    else:

is in fact the fastest, and beats if x != 'val': by ~0.3%. This isn't very readable, but I guess if you wanted a negligible performance improvement, one could go down this route.

I want to expand on my readability comment above.

Again, I completely agree with readability overriding other (performance-insignificant) concerns.

What I would like to point out is the brain interprets "positive" faster than it does "negative". E.g., "stop" vs. "do not go" (a rather lousy example due to the difference in number of words).

So given a choice:

if a == b
    (do this)
else
    (do that)

is preferable to the functionally-equivalent:

if a != b
    (do that)
else
    (do this)

Less readability/understandability leads to more bugs. Perhaps not in initial coding, but the (not as smart as you!) maintenance changes...

>>> from dis import dis
>>> dis(compile('not 10 == 20', '', 'exec'))
  1           0 LOAD_CONST               0 (10)
              3 LOAD_CONST               1 (20)
              6 COMPARE_OP               2 (==)
              9 UNARY_NOT
             10 POP_TOP
             11 LOAD_CONST               2 (None)
             14 RETURN_VALUE
>>> dis(compile('10 != 20', '', 'exec'))
  1           0 LOAD_CONST               0 (10)
              3 LOAD_CONST               1 (20)
              6 COMPARE_OP               3 (!=)
              9 POP_TOP
             10 LOAD_CONST               2 (None)
             13 RETURN_VALUE

Here you can see that not x == y has one more instruction than x != y. So the performance difference will be very small in most cases unless you are doing millions of comparisons and even then this will likely not be the cause of a bottleneck.

An additional note, since the other answers answered your question mostly correctly, is that if a class only defines __eq__() and not __ne__(), then your COMPARE_OP (!=) will run __eq__() and negate it. At that time, your third option is likely to be a tiny bit more efficient, but should only be considered if you NEED the speed, since it's difficult to understand quickly.