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

[אלעזר]

בתאריך יום ו׳, 14 באוק' 2016, 12:19, מאת Michel Desmoulin ‏<
desmoulinmichel at gmail.com>:

> Regarding all those examples:
>
> Le 14/10/2016 à 00:08, אלעזר a écrit :
> > Trying to restate the proposal, somewhat more formal following Random832
> > and Paul's suggestion.
> >
> > I only speak about the single star.
> > ---
> >
> > *The suggested change of syntax:*
> >
> >     comprehension ::=  starred_expression comp_for
> >
> > *Semantics:*
> >
> > (In the following, f(x) must always evaluate to an iterable)
> >
> > 1. List comprehension:
> >
> >     result = [*f(x) for x in iterable if cond]
> >
> > Translates to
> >
> >     result = []
> >     for x in iterable:
> >         if cond:
> >             result.extend(f(x))
> >
> > 2. Set comprehension:
> >
> >     result = {*f(x) for x in iterable if cond}
> >
> > Translates to
> >
> >     result = set()
> >     for x in iterable:
> >         if cond:
> >             result.update(f(x))
>
> Please note that we already have a way to do those. E.G:
>
>     result = [*f(x) for x in iterable if cond]
>
> can currently been expressed as:
>
>     >>> iterable = range(10)
>     >>> f = lambda x: [x] * x
>     >>> [y for x in iterable if x % 2 == 0 for y in f(x)]
>     [2, 2, 4, 4, 4, 4, 6, 6, 6, 6, 6, 6, 8, 8, 8, 8, 8, 8, 8, 8]
>
>
> Now I do like the new extension syntax. I find it more natural, and more
> readable:
>
>     >>> [*f(x) for x in iterable if x % 2 == 0]
>
> But it's not a missing feature, it's really just a (rather nice)
> syntaxic improvement.
>

It is about lifting restrictions from an existing syntax. That this
behavior is being *explicitly disabled* in the implementation is a strong
evidence, in my mind.

(There are more restrictions I was asked not to divert this thread, which
makes sense)

Elazar
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