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Title: __len__ called twice in the list() constructor
Type: performance Stage: resolved
Components: Interpreter Core Versions: Python 3.11, Python 3.10, Python 3.9
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: pablogsal Nosy List: Crowthebird, Dennis Sweeney, brett.cannon, eric.snow, godlygeek, kimiguel, methane, pablogsal, rhettinger, terry.reedy
Priority: normal Keywords: patch

Created on 2020-03-02 15:36 by kimiguel, last changed 2022-04-11 14:59 by admin. This issue is now closed.

Pull Requests
URL Status Linked Edit
PR 31816 merged Crowthebird, 2022-03-11 09:37
Messages (13)
msg363186 - (view) Author: Kim-Adeline Miguel (kimiguel) Date: 2020-03-02 15:36
(See #33234)

Recently we added Python 3.8 to our CI test matrix, and we noticed a possible backward incompatibility with the list() constructor.

We found that __len__ is getting called twice, while before 3.8 it was only called once.

Here's an example:

class Foo:
 def __iter__(self):
  return iter([3, 5, 42, 69])

 def __len__(self):
  return 4

Calling list(Foo()) using Python 3.7 prints:


But calling list(Foo()) using Python 3.8 prints:


It looks like this behaviour was introduced for #33234 with PR GH-9846. 

We realize that this was merged a while back, but at least we wanted to make the team aware of this change in behaviour.
msg363188 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2020-03-02 15:44
Why should that be backwards incompatible? The number of times we can `__len__` on the constructor is an implementation detail. The reason is called now twice is because there is an extra check for the preallocation logic, which is detached from the logic of the subsequent list_extend(self, iterable). 

On the other hand, there may be a chance for optimization here, but on a very rough first plan, that may require coupling some logic (passing down the calculated length to list_extend() or some helper, which I am not very fond of.
msg363588 - (view) Author: Terry J. Reedy (terry.reedy) * (Python committer) Date: 2020-03-07 10:23
The only specification is that len(ob) calls ob.__len__ and that ob.__len__ should return an 'integer >= 0'.  (Adding side effects goes beyond that spec.)  I agree that a detectable internal in list is not a bug.  Unless there is a realistic performance enhancement in caching the result of the first call, this issue should be closed.
msg363745 - (view) Author: Eric Snow (eric.snow) * (Python committer) Date: 2020-03-09 16:09
FWIW, I encouraged Kim to file this.  Thanks Kim!

While it isn't part of any specification, it is an unexpected change in behavior that led to some test failures.  So I figured it would be worth bringing up. :)  I did find it surprising that we were not caching the result, but don't think that's necessarily a problem.

All that said, the change did not actually break anything other than some tests (not the code they were testing).  So I don't have a problem with closing this.
msg363746 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2020-03-09 16:12
Thanks Kim and Eric!

I think it still makes sense to do some quick benchmarking and research on passing down the calculated length. I can try to produce a draft PR so we can discuss with something more tangible.
msg363747 - (view) Author: Eric Snow (eric.snow) * (Python committer) Date: 2020-03-09 16:20
I'm not opposed. :)  I just don't want to impose on your time.
msg414724 - (view) Author: Matt Wozniski (godlygeek) * Date: 2022-03-08 05:17
Pardon me for necroing an old issue, but someone pointed out the surprising behavior of `__len__` being called twice by `list(iterable)`, and it caught my curiosity. made it so that `list.__init__(iterable)` calls `iterable.__len__()` before calling `list.extend()`, to preallocate exactly the right amount of space, rather than allowing `list.extend()` to grow the array. That's because `list.extend()` can over-allocate.

What if instead, we made it so that `list.extend(iterable)` doesn't over-allocate when called on an empty list? In the two places where `list_extend` calls `list_resize` to grow the array, we'd check if `self->ob_item == NULL` and if so call `list_preallocate_exact` instead, and we'd remove the call to `list_preallocate_exact` from `list___init___impl`.

It seems like that ought to achieve the same goal as making `__init__` call preallocate exactly, without requiring the extra call to `__len__`.
msg414727 - (view) Author: Dennis Sweeney (Dennis Sweeney) * (Python committer) Date: 2022-03-08 06:02
Related to Matt's idea is
msg414840 - (view) Author: Jeremiah Gabriel Pascual (Crowthebird) * Date: 2022-03-10 11:05
Matt's idea leads to some speedups when implemented correctly (pardon me but I have no idea how to use pyperf):

list({}): Mean +- std dev: [orig] 109 ns +- 1 ns -> [modif] 103 ns +- 1 ns: 1.06x faster
list({1: 2}): Mean +- std dev: [orig] 125 ns +- 1 ns -> [modif] 118 ns +- 1 ns: 1.05x faster
list({(1, 2, 3): 4}): Mean +- std dev: [orig] 125 ns +- 1 ns -> [modif] 118 ns +- 1 ns: 1.05x faster
list((3, 3, 4)): Mean +- std dev: [orig] 89.2 ns +- 4.5 ns -> [modif] 82.9 ns +- 4.6 ns: 1.08x faster
list(()): Mean +- std dev: [orig] 70.1 ns +- 0.8 ns -> [modif] 65.5 ns +- 0.8 ns: 1.07x faster
list({0, 1, 2, ...}): Mean +- std dev: [orig] 74.7 us +- 3.6 us -> [modif] 67.6 us +- 1.6 us: 1.11x faster
list({9, 3}): Mean +- std dev: [orig] 131 ns +- 2 ns -> [modif] 126 ns +- 4 ns: 1.04x faster
list(set()): Mean +- std dev: [orig] 115 ns +- 6 ns -> [modif] 110 ns +- 2 ns: 1.05x faster
list([]): Mean +- std dev: [orig] 73.2 ns +- 5.5 ns -> [modif] 67.8 ns +- 3.4 ns: 1.08x faster
list([1, 2, 1, 1]): Mean +- std dev: [orig] 93.5 ns +- 9.8 ns -> [modif] 87.9 ns +- 8.6 ns: 1.06x faster
list([1, 2, 1, 2, 1, 2]): Mean +- std dev: [orig] 93.0 ns +- 3.1 ns -> [modif] 87.0 ns +- 2.7 ns: 1.07x faster

Benchmark hidden because not significant (3): list({0: 0, 1: ...}), list((4, 5, 1, ...)), list([4, 1, 3, ...])

Geometric mean: 1.05x faster

Changes compared here:
msg414889 - (view) Author: Inada Naoki (methane) * (Python committer) Date: 2022-03-11 05:54
> Changes compared here:

Looks good to me. Would you create a pull request?
msg415026 - (view) Author: Jeremiah Gabriel Pascual (Crowthebird) * Date: 2022-03-13 05:11
> Looks good to me. Would you create a pull request?

Created a pull request (31816).
msg415111 - (view) Author: Inada Naoki (methane) * (Python committer) Date: 2022-03-14 01:24
New changeset 2153daf0a02a598ed5df93f2f224c1ab2a2cca0d by Crowthebird in branch 'main':
bpo-39829: Fix `__len__()` is called twice in list() constructor (GH-31816)
msg415112 - (view) Author: Inada Naoki (methane) * (Python committer) Date: 2022-03-14 01:24
Date User Action Args
2022-04-11 14:59:27adminsetgithub: 84010
2022-03-14 01:24:38methanesetmessages: + msg415112
2022-03-14 01:24:29methanesetstatus: open -> closed
resolution: fixed
stage: patch review -> resolved
2022-03-14 01:24:06methanesetmessages: + msg415111
2022-03-13 23:42:35iritkatrielsetversions: - Python 3.8
2022-03-13 23:35:48Crowthebirdsetversions: + Python 3.10, Python 3.11
2022-03-13 05:11:02Crowthebirdsetmessages: + msg415026
2022-03-11 09:37:10Crowthebirdsetkeywords: + patch
stage: patch review
pull_requests: + pull_request29914
2022-03-11 05:54:15methanesetmessages: + msg414889
2022-03-10 11:05:28Crowthebirdsetnosy: + Crowthebird
messages: + msg414840
2022-03-08 06:36:11methanesetnosy: + methane
2022-03-08 06:02:46Dennis Sweeneysetnosy: + Dennis Sweeney
messages: + msg414727
2022-03-08 05:17:04godlygeeksetnosy: + godlygeek
messages: + msg414724
2020-03-09 16:20:31eric.snowsetstatus: closed -> open
messages: + msg363747

assignee: pablogsal
resolution: not a bug -> (no value)
stage: resolved -> (no value)
2020-03-09 16:12:29pablogsalsetmessages: + msg363746
2020-03-09 16:09:26eric.snowsetstatus: open -> closed
resolution: not a bug
messages: + msg363745

stage: resolved
2020-03-07 10:23:44terry.reedysettype: behavior -> performance

messages: + msg363588
nosy: + terry.reedy
2020-03-02 15:44:00pablogsalsetmessages: + msg363188
2020-03-02 15:36:16kimiguelcreate