[Python-ideas] Fwd: Fwd: unpacking generalisations for list comprehension

[Martti Kühne]

On Sat, Oct 15, 2016 at 10:09 AM, Steven D'Aprano <steve at pearwood.info> wrote:
> Not everything is a function. What's your point?
>
> As far as I can see, in *every* other use of sequence unpacking, *t is
> conceptually replaced by a comma-separated sequence of items from t. If
> the starred item is on the left-hand side of the = sign, we might call
> it "sequence packing" rather than unpacking, and it operates to collect
> unused items, just like *args does in function parameter lists.
>


You brush over the fact that *t is not limited to a replacement by a
comma-separated sequence of items from t, but *t is actually a
replacement by that comma-separated sequence of items from t INTO an
external context. For func(*t) to work, all the elements of t are kind
of "leaked externally" into the function argument list's context, and
for {**{'a': 1, 'b': 2, ...}} the inner dictionary's items are kind of
"leaked externally" into the outer's context.

You can think of the */** operators as a promotion from append to
extend, but another way to see this is as a promotion from yield to
yield from. So if you want to instead of append items to a
comprehension, as is done with [yield_me for yield_me in iterator],
you can see this new piece as a means to [*yield_from_me for
yield_from_me in iterator]. FWIW, I think it's a bit confusing
that yield needs a different keyword if these asterisk operators
already have this outspoken promotion effect.

Besides, [*thing for thing in iterable_of_iters if cond] has this cool
potential for the existing any() and all() builtins for cond, where a
decision can be made based on the composition of the in itself
iterable thing.

cheers!
mar77i