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

[Paul Moore]

On 14 October 2016 at 10:48, Paul Moore <p.f.moore at gmail.com> wrote:
> On 14 October 2016 at 07:54, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
>>> I think it's probably time for someone to
>>> describe the precise syntax (as BNF, like the syntax in the Python
>>> docs at
>>> https://docs.python.org/3.6/reference/expressions.html#displays-for-lists-sets-and-dictionaries
>>
>>
>> Replace
>>
>>    comprehension ::=  expression comp_for
>>
>> with
>>
>>    comprehension ::=  (expression | "*" expression) comp_for
>>
>>> and semantics (as an explanation of how to
>>> rewrite any syntactically valid display as a loop).
>>
>>
>> The expansion of the "*" case is the same as currently except
>> that 'append' is replaced by 'extend' in a list comprehension,
>> 'yield' is replaced by 'yield from' in a generator
>> comprehension.
[...]
> So now I understand what's being proposed, which is good. I don't
> (personally) find it very intuitive, although I'm completely capable
> of using the rules given to establish what it means. In practical
> terms, I'd be unlikely to use or recommend it - not because of
> anything specific about the proposal, just because it's "confusing". I
> would say the same about [(x, *y, z) for ...].

Thinking some more about this, is it not true that

[ *expression for var in iterable ]

is the same as

[ x for var in iterable for x in expression ]

?

If so, then this proposal adds no new expressiveness, merely a certain
amount of "compactness". Which isn't necessarily a bad thing, but it's
clearly controversial whether the compact version is more readable /
"intuitive" in this case. Given the lack of any clear improvement, I'd
be inclined to think that "explicit is better than implicit" applies
here, and reject the new proposal.

Paul.