On Sat, Oct 16, 2021 at 12:21 PM David Mertz, Ph.D. <david.mertz@gmail.com> wrote:
I'm not making any claims about tuple creation speed vs. list creation on microbenchmarks. It might we'll be 10% faster to create a million item tuple than a million item list. Or maybe the opposite, I don't know.

The thing to know about this (for everyone) is that creating a new tuple of known size requires a single allocation while creating a new list always requires two allocations. Where this really makes a difference is not when you have a million items but when you create thousands or millions of objects of modest size -- for lists it takes twice as many allocations.

If the number of items is not known there are different strategies depending on what API you use; interestingly, the strategy deployed by PySequence_Tuple() has a comment claiming it "can grow a bit faster than for lists because unlike lists the over-allocation isn't permanent."

Finally, the bytecode generated for (*a, *b) creates a list first and then turns that into a tuple (which will be allocated with the right size since it's known at that point).
--Guido van Rossum (python.org/~guido)