weirdness with list()

[Cameron Simpson]

On 01Mar2021 00:28, Peter Otten <__peter__ at web.de> wrote:
>On 28/02/2021 23:33, Marco Sulla wrote:
>>I can't reproduce, Am I missing something?
>>
>>marco at buzz:~$ python3
>>Python 3.6.9 (default, Jan 26 2021, 15:33:00)
>>[GCC 8.4.0] on linux
>>Type "help", "copyright", "credits" or "license" for more information.
>>>>>class A:
>>...     def __len__(self):
>>...             return 1024**3
>>...     def __iter__(self):
>>...             yield from ()
>>...
>>>>>a = A()
>>>>>len(a)
>>1073741824
>>>>>list(a)
>>[]
>>>>>
>>
>>It takes milliseconds to run list(a)
>
>Looks like you need at least Python 3.8 to see this. Quoting
>https://docs.python.org/3/whatsnew/3.8.html:
>"""
>The list constructor does not overallocate the internal item buffer if
>the input iterable has a known length (the input implements __len__).
>This makes the created list 12% smaller on average. (Contributed by
>Raymond Hettinger and Pablo Galindo in bpo-33234.)
>"""

That may also explain why I hadn't noticed this before, eg last year.

I do kind of wish __length_hint__ overrode __len__ rather than the other 
way around, if it's doing what I think it's doing.

Cheers,
Cameron Simpson <cs at cskk.id.au>