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
<mailto: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.
Best wishes
Rob Cliffe
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