sum() vs. loop

Dan Stromberg drsalists at
Mon Oct 11 23:41:35 EDT 2021

On Mon, Oct 11, 2021 at 2:54 PM Steve Keller <keller.steve at> wrote:

> I have found the sum() function to be much slower than to loop over the
> operands myself:
> def sum_products(seq1, seq2):
>     return sum([a * b for a, b in zip(seq1, seq2)])
> def sum_products2(seq1, seq2):
>     sum = 0
>     for a, b in zip(seq1, seq2):
>         sum += a * b
>     return sum
> In a program I generate about 14 million pairs of sequences of ints each
> of length 15 which need to be summed.  The first version with sum() needs
> 44 seconds while the second version runs in 37 seconds.
> Can someone explain this difference?
I can't explain it.  It might help to try a line-by-line profiler.

If you need speed, maybe try Cython, numpy and/or numba.

It seems like the generator expression should be the fastest to me.  But
writing for speed instead of writing for clarity is usually not a great

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