On Sat, Oct 15, 2016 at 9:42 PM Steven D'Aprano <st...@pearwood.info> wrote:

> On Sun, Oct 16, 2016 at 12:48:36PM +1300, Greg Ewing wrote:
> > Steven D'Aprano wrote:
> > >Are you now supporting my argument that starring the list comprehension
> > >expression isn't meaningful?
> >
> > The context it's in (a form of list display) has a clear
> > meaning for a comma-separated list of values, so there
> > is a reasonable interpretation that it *could* be given.
> This thread is a huge, multi-day proof that people do not agree that
> this is a "reasonable" interpretation.
> > >py> iterable = [(1, 'a'), (2, 'b')]
> > >py> [(100, *t) for t in iterable]
> > >[(100, 1, 'a'), (100, 2, 'b')]
> >
> > The * there is in the context of constructing a tuple,
> > not the list into which the tuple is placed.
> Right: the context of the star is meaningful. We all agree that *t in a
> list display [a, b, c, ...] is meaningful; same for tuples; same for
> function calls; same for sequence unpacking for assignment.
> What is not meaningful (except as a Perlish line-noise special case to
> be memorised) is *t as the list comprehension expression.
> I've never disputed that we could *assert* that *t in a list comp means
> "flatten". We could assert that it means anything we like. But it
> doesn't follow from the usual meaning of sequence unpacking anywhere
> else -- that's why it is currently a SyntaxError, and that's why people
> reacted with surprise at the OP who assumed that *t would magically
> flatten his iterable. Why would you assume that? It makes no sense to me
> -- that's not how sequence unpacking works in any other context, it
> isn't how list comprehensions work.
> Right from the beginning I called this "wishful thinking", and *nothing*
> since then has changed my mind. This proposal only makes even a little
> bit of sense if you imagine list comprehensions
>     [*t for a in it1 for b in it2 for c in it3 ... for t in itN]
> completely unrolled into a list display:
>     [*t, *t, *t, *t, ... ]
> but who does that? Why would you reason about your list comps like that?
> If you think about list comps as we're expected to think of them -- as
> list builders equivalent to a for-loop -- the use of *t there is
> invalid. Hence it is a SyntaxError.
> You want a second way to flatten your iterables? A cryptic, mysterious,
> Perlish line-noise way? Okay, fine, but don't pretend it is sequence
> unpacking -- in the context of a list comprehension, sequence unpacking
> doesn't make sense, it is invalid. Call it something else: the new
> "flatten" operator:
>     [^t for t in iterable]
> for example, which magically adds an second invisible for-loop to your
> list comps:
>     # expands to
>     for t in iterable:
>         for x in t:
>             result.append(x)
> Because as your own email inadvertently reinforces, if sequence
> unpacking made sense in the context of a list comprehension, it would
> already be allowed rather than a SyntaxError: it is intentionally
> prohibited because it doesn't make sense in the context of list comps.

Whoa, hang on a second there.  It is intentionally prohibited because
Joshua Landau (who helped a lot with writing and implementing the PEP) and
I felt like there was going to be a long debate and we wanted to get PEP
448 checked in.

If it "didn't make sense" as you say, then we would have said so in the
PEP. Instead, Josh wrote:

This was met with a mix of strong concerns about readability and mild
support. In order not to disadvantage the less controversial aspects of the
PEP, this was not accepted with the rest of the proposal.

I don't remember who it was who had those strong concerns (maybe you?)  But
that's why we didn't include it.



> --
> Steve
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