[Python-ideas] Generator unpacking

[Michael Selik]

On Fri, Feb 12, 2016 at 6:01 PM Erik <python at lucidity.plus.com> wrote:

> On 12/02/16 19:54, Andrew Barnert via Python-ideas wrote:
> > Meanwhile, you can always write the expanded version out explicitly.
> (And you can leave off the first line when you know c is already an
> iterator.) Or you can use itertools.islice to make it more compact:
> >
> >      >>> a, b = itertools.islice(c, 2)
> >      >>> rest = c
>
> Why not just have an itertools.unpack() - a simple version without
> argument checking:
>
> def unpack(seq, num):
>    it = iter(seq)
>    yield from (i[1] for i in zip(range(num), it))
>    yield it
>
> foo, bar, rest = unpack([1, 2, 3, 4, 5, 6], 2)
>
> Because it's in itertools, the expectation is that it has something to
> do with iterators so the final return value always being an iterator
> regardless of the original sequence type is reasonable (and is perhaps
> the only justification for putting it in itertools in the first place ;) ).


There's some visual dissonance since the ``num`` argument is asking for the
number of elements to unpack, but the left-hand of the assignment has num+1
variables.

What do you think about just using islice?

    >>> from itertools import islice
    >>> it = iter(range(5))
    >>> (first, second), rest = islice(it, 0, 2), it
    >>> first
    0
    >>> second
    1
    >>> rest
    <range_iterator object at 0x1011944b0>

I suppose it might read better broken apart into two lines to emphasize
that the state changed.

    >>> first, second = islice(it, 0, 2)
    >>> rest = it
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20160212/8e79c66b/attachment.html>