Pythonification of the asterisk-based collection packing/unpacking syntax

[Eelco]

On Dec 25, 2:13 pm, Steven D'Aprano <steve
+comp.lang.pyt... at pearwood.info> wrote:
> On Sat, 24 Dec 2011 06:47:21 -0800, Eelco wrote:
> > I would like to be able to write something like:
>
> > a, middle::tuple, b = ::sequence
>
> > Where I would like the extra :: before the sequence to explicitly signal
> > collection unpacking on the rhs, to maintain the symmetry with
> > collection unpacking within a function call.
>
> The :: on the right-hand side is redundant, because the left-hand side
> already explicitly signals collection unpacking of the RHS. Requiring ::
> on the RHS above is as unnecessary as it would be here:

Yes, it is redundant; hence the word 'extra' in my post.

Explicit and implicit are not well-defined terms, but I would say that
at the moment the signal is implicit, in the sense that one cannot see
what is going on by considering the rhs in isolation. Normally in
python, an assignment just binds the rhs to the identifiers on the
lhs, but in case of collection (un)packing, this rule that holds
almost all of the time is broken, and the assignment statement implies
a far more complicated construct, with a far more subtle meaning, and
non-constant time complexity.

Thats not a terrible thing, but a little extra explicitness there
would not hurt, and like I argued many times before, it is a nice
unification with the situation where the unpacking can not be
implicit, like inside a function call rather than assignment.


>     n = len(::sequence)

Now you are just discrediting yourself in terms of having any idea
what you are talking about.