[Python-3000] PEP 3132: Extended Iterable Unpacking

[Brett Cannon]

On 5/1/07, Guido van Rossum <guido at python.org> wrote:
>
> On 5/1/07, Georg Brandl <g.brandl at gmx.net> wrote:
> > This is a bit late, but it was in my queue by April 30, I swear! ;)
>
> Accepted.
>
> > Comments are appreciated, especially some phrasing sounds very clumsy
> > to me, but I couldn't find a better one.
> >
> > Georg
> >
> >
> > PEP: 3132
> > Title: Extended Iterable Unpacking
> > Version: $Revision$
> > Last-Modified: $Date$
> > Author: Georg Brandl <georg at python.org>
> > Status: Draft
> > Type: Standards Track
> > Content-Type: text/x-rst
> > Created: 30-Apr-2007
> > Python-Version: 3.0
> > Post-History:
> >
> >
> > Abstract
> > ========
> >
> > This PEP proposes a change to iterable unpacking syntax, allowing to
> > specify a "catch-all" name which will be assigned a list of all items
> > not assigned to a "regular" name.
> >
> > An example says more than a thousand words::
> >
> >     >>> a, *b, c = range(5)
> >     >>> a
> >     0
> >     >>> c
> >     4
> >     >>> b
> >     [1, 2, 3]
>
> Has it been pointed out to you already that this particular example is
> hard to implement if the RHS is an iterator whose length is not known
> a priori? The implementation would have to be quite hairy -- it would
> have to assign everything to the list b until the iterator is
> exhausted, and then pop a value from the end of the list and assign it
> to c. it would be much easier if *b was only allowed at the end. (It
> would be even worse if b were assigned a tuple instead of a list, as
> per your open issues.)



If a clean implementation solution cannot be found then I say go with the
last-item-only restriction.  You still get the nice functional language
feature of car/cdr (or x:xs if you prefer ML or Haskell) without the
implementation headache.  I mean how often do you want the head and tail
with everything in between left together?  If I needed that kind of sequence
control I would feed the iterator to a list comp and get to the items that
way.

Also, what should this do? Perhaps the grammar could disallow it?
>
> *a = range(5)



I say disallow it.  That is ambiguous as to what your intentions are even if
you know what '*' does for multiple assignment.

-Brett
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