weirdness with list()

[Marco Sulla]

On Sun, 28 Feb 2021 at 01:19, Cameron Simpson <cs at cskk.id.au> wrote:
> My object represents an MDAT box in an MP4 file: it is the ludicrously
> large data box containing the raw audiovideo data; for a TV episode it
> is often about 2GB and a movie is often 4GB to 6GB.
> [...]
> That length is presented via the object's __len__ method
> [...]
>
> I noticed that it was stalling, and investigation revealed it was
> stalling at this line:
>
>     subboxes = list(self)
>
> when doing the MDAT box. That box (a) has no subboxes at all and (b) has
> a very large __len__ value.
>
> BUT... It also has a __iter__ value, which like any Box iterates over
> the subboxes. For MDAT that is implemented like this:
>
>     def __iter__(self):
>         yield from ()
>
> What I was expecting was pretty much instant construction of an empty
> list. What I was getting was a very time consuming (10 seconds or more)
> construction of an empty list.

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)