If you know the input is sizeable, why not check its length instead of the map's?
On Mon, Nov 26, 2018, 1:35 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: > > class map: > > def __init__(self, func, iterable): > self.func = func > self.iterable = iterable > > def __iter__(self): > yield from (self.func(x) for x in self.iterable) > > def __len__(self): > return len(self.iterable) > > def __getitem__(self, key): > return self.func(self.iterable[key]) > > Let me know if there any downsides to this that I'm not seeing. From my > perspective, it seems like there would be only a number of (small) > advantages: > > - Less surprising > - Avoid some unnecessary copies > - Backwards compatible > > -Kale > _______________________________________________ > 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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