The list() constructor isn't taking full advantage of known input lengths or length hints.  Ideally, it should pre-size and not over-allocate when the input size is known or can be reasonably estimated.Python 3.8.0a0 (heads/master:091e95e900, Apr  5 2018, 09:48:33)[Clang 9.1.0 (clang-902.0.39.1)] on darwinType "help", "copyright", "credits" or "license" for more information.>>> from sys import getsizeof>>> getsizeof([0] * 10)144>>> getsizeof(list([0] * 10))200>>> getsizeof(list(range(10)))200

Calling PyObject_LengthHint() adds an overhead. It is significant for short sequences. I work on a patch that reduces it.PR 6493 adds the second call of PyObject_LengthHint() and increases the overhead.As for this issue, in-place repeat overallocates too.>>> a = [0]; a *= 10; getsizeof(a)200I think it would be better to make it not preallocating.And maybe it would be worth to avoid overallocating if newsize > allocated + allocated/8 or something like.

Microbenchmarked with @vstinner's perf module:python -m perf timeit "list([0]*10)" -o new.jsoncheckout master and buildpython -m perf timeit "list([0]*10)" -o old.jsonpython -m perf compare_to new.json old.jsonMean +- std dev: [new] 241 ns +- 3 ns -> [old] 243 ns +- 18 ns: 1.01x slower (+1%)Not significant!

See alsohttp://bugs.python.org/issue26814#msg264003 andbpo-26828.

Should we shrink the list of the length hint was way too big? For example, if the length hint was 100 but the final list of only 10. Should we shrink in that case?I'm asking because it seems like Pablo's PR doesn't shrink the list in that case.I wasn't sure so I checked, del shrinks the list if needed:>>> x=list(range(1000))>>> import sys>>> sys.getsizeof(x)9112>>> del x[1:]>>> sys.getsizeof(x)96

@serhiy.storchaka @rhettinger @vstinner Should we better make the pre-allocation if the length of the iterable is known (so we call PyObject_Length and not PyObject_LengthHint)? This will keep the logic simpler (so not shrinking if PyObject_LengthHint gives more than the real length) and it will not be as expensive as calling PyObject_LengthHint.

New changeset372d705d958964289d762953d0a61622755f5386 by Pablo Galindo in branch 'master':bpo-33234 Improve list() pre-sizing for inputs with known lengths (GH-9846)https://github.com/python/cpython/commit/372d705d958964289d762953d0a61622755f5386

>bpo-33234 Improve list() pre-sizing for inputs with known lengths (GH-9846)Oh. Can you please document the optimization in the following doc as well?https://docs.python.org/dev/whatsnew/3.8.html#optimizations

>Oh. Can you please document the optimization in the following doc as well?Sure! Openedhttps://github.com/python/cpython/pull/10200 to address that. :)

New changesetc61e229d2a4c54ffb4153e1f0f48126ba33c9cbf by Pablo Galindo in branch 'master':bpo-33234: Add exact allocation optimization to lists in What's New (GH-10200)https://github.com/python/cpython/commit/c61e229d2a4c54ffb4153e1f0f48126ba33c9cbf

New changeset0e5f771f38138714415f665651de7e674fcebc38 by Pablo Galindo (Sergey Fedoseev) in branch 'master':bpo-33234: Simplify list_preallocate_exact() (GH-11220)https://github.com/python/cpython/commit/0e5f771f38138714415f665651de7e674fcebc38

New changesete182318e6a473e6e9341f1aab8e49f2b1035c674 by Miss Islington (bot) (Raymond Hettinger) in branch 'master':bpo-33234: Add another attribution in Whatsnew (GH-11899)https://github.com/python/cpython/commit/e182318e6a473e6e9341f1aab8e49f2b1035c674

Possible backward incompatibility caused by this issue:issue39829

See alsobpo-14126: "Speed up list comprehensions by preallocating the list where possible".