On Sat, Jun 19, 2021 at 5:13 PM Rob Cliffe via Python-ideas
<python-ideas@python.org> wrote:
>
>
>
> On 19/06/2021 05:33, Guido van Rossum wrote:
>
> 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.
>
> I'm not convinced.
> 1, 2 + x
> does not mean the same as
> chunk + x
> but it would be if there were parentheses around the "1, 2". The same
> applies to your example.
A closer equivalent would be:
*chunk + x
which is illegal.
ChrisA
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