On Mon, Oct 17, 2016 at 12:11:46PM -0400, Random832 wrote:
> Honestly, it goes beyond just being "wrong". The repeated refusal to
> even acknowledge any equivalence between [...x... for x in [a, b, c]]
> and [...a..., ...b..., ...c...] truly makes it difficult for me to
> accept some people's _sincerity_.
While we're talking about people being insincere, how about if you take
a look at your own comments? This "repeated refusal" that you accuse us
(opponents of this proposal) of is more of a rhetorical fiction than an
actual reality. Paul, David and I have all acknowledged the point you
are trying to make. I won't speak for Paul or David, but speaking for
myself, it isn't that I don't understand the point you're trying to
make, but that I do not understand why you think that point is
meaningful or desirable.
I have acknowledged that starring the expression in a list comprehension
makes sense if you think of the comprehension as a fully unrolled list
[*expr, *expr *expr, *expr, ...]
What I don't believe is:
(1) that the majority of Python programmers (or even a large minority)
regularly and consistently think of comprehensions as syntactic sugar
for a completely unrolled list display; rather, I expect that they
usually think of them as sugar for a for-loop;
(2) that we should encourage people to think of comprehensions as sugar
for a completely unrolled list display rather than a for-loop;
(3) that we should accept syntax which makes no sense in the context
of a for-loop-with-append (i.e. the thing which comprehensions are sugar
But if we *do* accept this syntax, then I believe that we should drop
the pretense that it is a natural extension of sequence unpacking in the
context of a for-loop-with-append (i.e. list comprehensions) and accept
that it will be seen by people as a magical "flatten" operator.
And, in my opinion, rightly so: the semantic distance between *expr in a
list comp and the level of explanation where it makes sense is so great
that thinking of it as just special syntax for flattening is the
simplest way of looking at it.
So, yet again for emphasis: I see what you mean about unrolling the list
comprehension into a list display. But I believe that's not a helpful
way to think about list comprehensions. The way we should be thinking
about them is as for-loops with append, and in *that* context, sequence
unpacking doesn't make sense.
In a list comprehension, we expect the invariant that the number of
items produced will equal the number of loops performed. (Less if there
are any "if" clauses.) There is one virtual append per loop. You cannot
get the behaviour you want without breaking that invariant: either the
append has to be replaced by extend, or you have so insert an extra loop
into your mental picture of comprehensions.
Yet again, for emphasis: I understand that you don't believe that
invariant is important, or at least you are willing to change it. But
drop the pretense that this is an obvious extension to the well-
established behaviour of list comprehensions and sequence unpacking.
If you think you can convince people (particularly Guido) that this
flattening behaviour is important enough to give up the invariant "one
append per loop", then by all means try. For all I know, Guido might
agree with you and love this idea! But while you're accusing us of
refusing to acknowledge the point you make about unrolling the loop to a
list display (what I maintain is an unhelpful and non-obvious way of
thinking about this), you in turn seem to be refusing to acknowledge the
points we have made. This isn't a small change: it requires not
insignificant changes to people's understanding of what list
comprehension syntax means and does.
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