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:
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:
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.
In the first one, Python has to execute one more operation 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
>>> 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.
@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...
Using
dis
to look at the bytecode generated for the two versions:not ==
!=
The latter has fewer operations, and is therefore likely to be slightly more efficient.
It was pointed out in the commments (thanks, @Quincunx) that where you have
if foo != bar
vs.if not foo == bar
the number of operations is exactly the same, it's just that theCOMPARE_OP
changes andPOP_JUMP_IF_TRUE
switches toPOP_JUMP_IF_FALSE
:not ==
:!=
In this case, unless there was a difference in the amount of work required for each comparison, it's unlikely you'd see any performance difference at all.
However, note that the two versions won't always be logically identical, as it will depend on the implementations of
__eq__
and__ne__
for the objects in question. Per the data model documentation:For example:
Finally, and perhaps most importantly: in general, where the two are logically identical,
x != y
is much more readable thannot x == y
.