That would be great especially if it returned objects of a subclass of map so that it didn't break any code that checks isinstance, however; I think this goes a little beyond map. I've run into cases using itertools where I wished the iterators could support len. I suppose you could turn those all into factories too, but I wonder if that's the most elegant solution.
On Thu, Nov 29, 2018 at 7:22 PM Paul Svensson <paul-pyt...@svensson.org> wrote: > On Mon, Nov 26, 2018 at 10:35 PM Kale Kundert <k...@thekunderts.net> > wrote: > > > > I just ran into the following behavior, and found it surprising: > > > >>>> len(map(float, [1,2,3])) > > TypeError: object of type 'map' has no len() > > > > I understand that map() could be given an infinite sequence and > therefore might not always have a length. But in this case, it seems like > map() should've known that its length was 3. I also understand that I can > just call list() on the whole thing and get a list, but the nice thing > about map() is that it doesn't copy data, so it's unfortunate to lose that > advantage for no particular reason. > > > > My proposal is to delegate map.__len__() to the underlying iterable. > Similarly, map.__getitem__() could be implemented if the underlying > iterable supports item access: > > > > Excellent proposal, followed by a flood of confused replies, > which I will mostly disregard, since all miss the obvious. > > What's being proposed is simple, either: > * len(map(f, x)) == len(x), or > * both raise TypeError > > That implies, loosely speaking: > * map(f, Iterable) -> Iterable, and > * map(f, Sequence) -> Sequence > > But, *not*: > * map(f, Iterable|Sequence) -> Magic. > > So, the map() function becomes a factory, returning an object > with __len__ or without, depending on what it was called with. > > /Paul > _______________________________________________ > Python-ideas mailing list > Python-ideas@python.org > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/ >
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