Things like progressbars want len() to work on iterated objects.

It’s possible to define __len__ for many of the iterables returned by itertools.

some arguments have to be iterated to find the len(): of course we have to check if those are reentrant, and raise a TypeError if they are non-reentrant. (signified by “(r)→”)

for the predicate functions, it’s questionable if we should offer it, since they might take a long time and “len” is a property-like function that feels like it should return fast.

map(func, iterable) → len(iterable)

count(), cycle(), repeat()  → infinty, but because len() returns integers, and there’s only float infinity, that’s impossible

accumulate(iterable)  →  len(iterable)
chain(*iterables)  →  sum(len(it) for it in iterables)
chain.from_iterable(iterables)  (r)→  like the above
compress(data, selectors)  (r)→  sum(1 for s in selectors if s)
dropwhile(pred, iterable)  (r)→  for skip, r in enumerate(map(pred, iterable)): if r: return len(iterable) - skip
filterfalse(pred, iterable)  (r)→  sum(1 for r in map(pred, iterable) if r)
groupby(iterable[, keyfunc])  (r)→  no way but to actually execute it all
islice(seq, [start,] stop [, step])  →  calculatable if len(seq) is possible
starmap(function, iterables)  →  len(iterables)
takewhile(pred, iterable)  (r)→  for skip, r in enumerate(map(pred, iterable)): if not r: return skip
tee(iterable[, n])  →  n
zip_longest(*iterables[, fillvalue])  (r)→  max(len(it) for it in iterables)


product(), permutations(), combinations(), combinations_with_replacement()  →  there’s math for that.