On Fri, Jun 18, 2021 at 8:40 PM Steven D'Aprano <st...@pearwood.info> wrote:
> On Fri, Jun 18, 2021 at 07:38:49AM -0700, Guido van Rossum wrote:
>
> > Note the ambiguity around whether the user might have meant
> >
> > [x,(y for y in a)]
> >
> > or
> >
> > [(x, y) for y in a]
>
> We already have a rule to disambiguate generator comprehensions: they
> must always be parenthesized unless they are already parenthised:
>
> g = (y for y in a) # parens required
> t = 999, (y for y in a) # parens required
>
> func((y for y in a)) # inner parens optional
>
Yes, that's exactly what I was referring to.
> That’s a good enough reason for me to also disallow *chunks.
>
> That's an odd way to look at it. We must disallow an unambiguous syntax
> because a completely different syntax would have been ambiguous if we
> didn't already have a rule in place that disambiguates it.
>
Maybe, but the two are not unrelated. In other contexts, when we accept
"*chunk", and 'chunk' equals "1, 2", we also accept "1, 2" in the original
position, and it means the same thing. This is useful as an explanation of
the semantics of the unary '*' operator[1]. For example
# Given:
chunk = 1, 2
# Equivalent:
f(*chunk)
f(1, 2)
# Equivalent:
[*chunk]
[1, 2]
So then if we were to allow this:
[*chunk for chunk in ...]
we ought to consider this equivalent:
[1, 2 for chunk in ...]
(Note there's nothing that says the expressions to the left of 'for' need
to involve the for-control variable 'chunk'. :-)
Now, this shouldn't be considered an airtight argument against [*chunk for
...], but it does show that there's no straightforward explanation of its
meaning through equivalence (like the OP seemed to think), and I think this
is what Serhiy was also getting at in his post.
__________
[1] Does the unary star operator have a name when used like this in Python?
In JavaScript the equivalent syntax ("...chunk", where the "..." are a
literal ellipsis) is called "spread". We could borrow this term.
--
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*
<http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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