On Sat, Oct 15, 2016 at 9:42 PM Steven D'Aprano <steve@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.

Best,

Neil



--
Steve
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