[Python-ideas] __len__() for map()

[Kale Kundert]

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

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20181126/242f4b80/attachment.html>