Python Performance Quirks: in vs. or

In Python, the standard way to check for equality is:

if a == b:
    ...

You would think that being explicit would be higher performance in Python, and when it comes to explicitness for multiple chained comparisons, there’s nothing that quite beats:

if a == b or a == c:
    ...

But it turns out that that’s not the highest performance way to code the comparison. This code does the same logical job using in and a tuple:

if a in (b, c):
    ...

Now, to my eye, that looks like it should perform worse. There’s a data structure that must be iterated over and a value compared to each iterated item from the data structure, whereas in the first example, we have two explicit and very simple operations for whcih no complex datastructures need to be read, unless a, b or c are more than trivial values, and yet in performance testing:

$ time python3 -c 'n = sum(x for x in range(1000000000) if x == 1 or x == 3); print(f"n={n}")'
n=4

real    0m38.234s
$ time python3 -c 'n = sum(x for x in range(1000000000) if x in (1,3)); print(f"n={n}")'
n=4

real    0m34.211s

The difference is not large, but it’s extremely consistent. The tuple version is approximately 10% faster than the two or‘ed comparisons… but how does this scale? Shockingly! When I changed the above code to include ... or x == 7, the tuple version became 20% faster than the or‘ed version! There’s clearly something about chained comparisons that’s not nearly as efficient as it should be!

But some good news for expected results: x == 1 is faster than x in (1,) by a very small amount… so there’s that.