`enumerate.__length_hint__`

[casperdcl]

Feature or enhancement

Proposal:

It would be nice if enumerate could expose the underlying len(iterable).

assert len(range(9)) == len(enumerate(range(9)))

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

As per tqdm#1185, there are 200k+ scripts using tqdm(enumerate(...)) on GitHub which would benefit from transparent len().

Somewhat related: #145361 and tqdm.contrib.itertools

/CC @KowalskiThomas @cmaloney @rhettinger

[cmaloney]

Reading through tqdm a bit it looks like this is to get a good progress bar / behavior by default in more cases?

I'm open to a __length_hint__ here if performance is flat to help with that. enumerate already has some other special pieces to improve performance such as implementing vectorcall protocol. There are a number of cases __length_hint__ can pessimize so it can't be added to iterators generators. To handle all the cases special-casing with match for types without it is likely going to be needed. To me a bit a standard way / Protocol to get the underlying seems more useful here.

Would be nice to figure out how to get length_hint to always be a win so could provide it as a default but that is very much a research project.

[rhettinger]

This was explored long ago and was rejected because it was not workable for the general case.

See Lib/test/test_iterlen.py.