[Python-Dev] Extending tuple unpacking

[Greg Ewing]

Guido van Rossum wrote:

> BTW, what should
> 
>     [a, b, *rest] = (1, 2, 3, 4, 5)
> 
> do? Should it set rest to (3, 4, 5) or to [3, 4, 5]?

Whatever type is chosen, it should be the same type, always.
The rhs could be any iterable, not just a tuple or a list.
Making a special case of preserving one or two types doesn't
seem worth it to me.

> Suppose the latter. Then should we allow
> 
>     [*rest] = x
> 
> as alternative syntax for
> 
>     rest = list(x)

That would be a consequence of that choice, yes, but so what?
There are already infinitely many ways of writing any expression.

> ? And then perhaps
> 
>     *rest = x
> 
> should mean
> 
>     rest = tuple(x)
> 
> Or should that be disallowed

Why bother? What harm would result from the ability to write that?

> There certainly is a need for doing the same from the end:
> 
>     *rest, a, b = (1, 2, 3, 4, 5)

I wouldn't mind at all if *rest were only allowed at the end.
There's a pragmatic reason for that if nothing else: the rhs
can be any iterable, and there's no easy way of getting "all
but the last n" items from a general iterable.

> Where does it stop?

For me, it stops with *rest only allowed at the end, and
always yielding a predictable type (which could be either tuple
or list, I don't care).

> BTW, and quite unrelated, I've always felt uncomfortable that you have to write
> 
>     f(a, b, foo=1, bar=2, *args, **kwds)
> 
> I've always wanted to write that as
> 
>     f(a, b, *args, foo=1, bar=2, **kwds)

Yes, I'd like that too, with the additional meaning that
foo and bar can only be specified by keyword, not by
position.

Greg