[Python-Dev] Any grammar experts?

[Antoine Pitrou]

On Mon, 26 Jan 2015 10:46:02 -0500
"R. David Murray" <rdmurray at bitdance.com> wrote:

> On Mon, 26 Jan 2015 09:43:26 -0500, Barry Warsaw <barry at python.org> wrote:
> > On Jan 25, 2015, at 09:31 PM, R. David Murray wrote:
> > 
> > >> > > {*x for x in it}
> > >> > >
> > >> > > which is a set comprehension, while the other is a dict comprehension :)
> > >> > >
> > >> > 
> > >> > That distinction doesn't bother me -- you might as well claim it's
> > >> > confusing that f(*x) passes positional args from x while f(**x) passes
> > >> > keyword args.
> > >> > 
> > >> > And the varargs set comprehension is similar to the varargs list
> > >> > comprehension:
> > >> > 
> > >> > [*x for x in it]
> > >> > 
> > >> > If `it` were a list of three items, this would be the same as
> > >> > 
> > >> > [*it[0], *it[1], *it[2]]
> > >> 
> > >> I find all this unreadable and difficult to understand.
> > >
> > >I did too, before reading the PEP.
> > >
> > >After reading the PEP, it makes perfect sense to me.  Nor is the PEP
> > >complicated...it's just a matter of wrapping your head around the
> > >generalization[*] of what are currently special cases that is going on
> > >here.
> > 
> > It does make sense after reading the PEP but it also reduces the readability
> > and instant understanding of any such code.  This is head-scratcher code that
> > I'm sure I'd get asked about from folks who aren't steeped in all the dark
> > corners of Python.  I don't know if that's an argument not to adopt the PEP,
> > but it I think it would be a good reason to recommend against using such
> > obscure syntax, unless there's a good reason (and good comments!).
> 
> But it is only obscure because we are not used to it yet.  It is a
> logical extension of Python's existing conventions once you think about
> it.  It will become "obvious" quickly, IMO.

I have to agree with Barry. While the following is obvious even without
having ever used it:

  a, b, *c = range(5)

the following makes me scratch my head and I can't seem to guess what
the *intent* is - which is very problematic when e.g. reviewing code:

  [*x for x in it]
  {**x for x in it}

  (and other similar things)

I also think the multiple-starargs function calls are completely
overboard:

  f(**someargs, **someotherargs)

(I might add I've never felt any need for those)

These generalizations look to me like a case of consistency taken too
far. When some construct needs commenting to be understandable, then
probably that construct shouldn't exist at all.

(yes, I should have reacted earlier. I hadn't realized the PEP
went that far. It seemed short and simple so I didn't look
carefully :-))

By the way, when reading https://www.python.org/dev/peps/pep-0448/, I
can't find a reference to the final pronouncement.

Regards

Antoine.