On Thu, Nov 29, 2018 at 08:13:12PM -0500, Paul Svensson wrote: > Excellent proposal, followed by a flood of confused replies, > which I will mostly disregard, since all miss the obvious.
When everyone around you is making technical responses which you think are "confused", it is wise to consider the possibility that it is you who is missing something rather than everyone else. > What's being proposed is simple, either: > * len(map(f, x)) == len(x), or > * both raise TypeError Simple, obvious, and problematic. Here's a map object I prepared earlier: from itertools import islice mo = map(lambda x: x, "aardvark") list(islice(mo, 3)) If I now pass you the map object, mo, what should len(mo) return? Five or eight? No matter which choice you make, you're going to surprise and annoy people, and there will be circumstances where that choice will introduce bugs into their code. > That implies, loosely speaking: > * map(f, Iterable) -> Iterable, and > * map(f, Sequence) -> Sequence But map objects aren't sequences. They're iterators. Just adding a __len__ method isn't going to make them sequences (not even "loosely speaking") or solve the problem above. In principle, we could make this work, by turning the output of map() into a view like dict.keys() etc, or a lazy sequence type like range(). wrapping the underlying sequence. That might be worth exploring. I can't think of any obvious problems with a view-like interface, but that doesn't mean there aren't any. I've spent like 30 seconds thinking about it, so the fact that I can't see any problems with it means little. But its also a big change, not just a matter of exposing the __len__ method of the underlying iterable (or iterables). -- Steve _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
