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

[אלעזר]

Steve, you only need to allow multiple arguments to append(), then it makes
perfect sense.

בתאריך יום ד׳, 12 באוק' 2016, 18:43, מאת Steven D'Aprano ‏<
steve at pearwood.info>:

> On Tue, Oct 11, 2016 at 02:42:54PM +0200, Martti Kühne wrote:
> > Hello list
> >
> > I love the "new" unpacking generalisations as of pep448. And I found
> > myself using them rather regularly, both with lists and dict.
> > Today I somehow expected that [*foo for foo in bar] was equivalent to
> > itertools.chain(*[foo for foo in bar]), which it turned out to be a
> > SyntaxError.
>
> To me, that's a very strange thing to expect. Why would you expect that
> unpacking items in a list comprehension would magically lead to extra
> items in the resulting list? I don't think that makes any sense.
>
> Obviously we could program list comprehensions to act that way if we
> wanted to, but that would not be consistent with the ordinary use of
> list comprehensions. It would introduce a special case of magical
> behaviour that people will have to memorise, because it doesn't follow
> logically from the standard list comprehension design.
>
> The fundamental design principle of list comps is that they are
> equivalent to a for-loop with a single append per loop:
>
>     [expr for t in iterable]
>
> is equivalent to:
>
>     result = []
>     for t in iterable:
>         result.append(expr)
>
>
> If I had seen a list comprehension with an unpacked loop variable:
>
>     [*t for t in [(1, 'a'), (2, 'b'), (3, 'c')]]
>
>
> I never in a million years would expect that running a list
> comprehension over a three-item sequence would magically expand to six
> items:
>
>     [1, 'a', 2, 'b', 3, 'c']
>
>
> I would expect that using the unpacking operator would give some sort
> of error, or *at best*, be a no-op and the result would be:
>
>     [(1, 'a'), (2, 'b'), (3, 'c')]
>
>
> append() doesn't take multiple arguments, hence a error should be the
> most obvious result. But if not an error, imagine the tuple unpacked to
> two arguments 1 and 'a' (on the first iteration), then automatically
> packed back into a tuple (1, 'a') just as you started with.
>
> I think it is a clear, obvious and, most importantly, desirable property
> of list comprehensions with a single loop that they cannot be longer
> than the initial iterable that feeds them. They might be shorter, if you
> use the form
>
>     [expr for t in iterable if condition]
>
> but they cannot be longer.
>
> So I'm afraid I cannot understand what reasoning lead you to
> expect that unpacking would apply this way. Wishful thinking
> perhaps?
>
>
>
>
> --
> Steve
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