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

[אלעזר]

On Thu, Oct 13, 2016 at 11:42 PM Paul Moore <p.f.moore at gmail.com> wrote:

> I remain puzzled.
>
> Given the well-documented and understood transformation:
>
> [fn(x) for x in lst if cond]
>
> translates to
>
> result = []
> for x in lst:
>    if cond:
>       result.append(fn(x))
>
> please can you explain how to modify that translation rule to
> incorporate the suggested syntax?
>

if you allow result.append(1, 2, 3) to mean result.extend([1,2,3])  # which
was discussed before

result = []
for x in lst:
   if cond:
      result.append(*fn(x))

Or simply use result.extend([*fn(x)])

Personally, I'm not even sure any more that I can *describe* the
> suggested syntax. Where in [fn(x) for x in lst if cond] is the *
> allowed? fn(*x)? *fn(x)? Only as *x with a bare variable, but no
> expression? Only in certain restricted types of construct which aren't
> expressions but are some variation on an unpacking construct?
>
>
The star is always exactly at the place that should "handle" it. which
means [*(fn(x)) for x in lst if cond]. fn(x) must be iterable as always.


> We've had a lot of examples. I think it's probably time for someone to
> describe the precise syntax (as BNF, like the syntax in the Python
> docs at
> https://docs.python.org/3.6/reference/expressions.html#displays-for-lists-sets-and-dictionaries
> and following sections) and semantics (as an explanation of how to
> rewrite any syntactically valid display as a loop). It'll have to be
> done in the end, as part of any implementation, so why not now?
>
>
I will be happy to do so, and will be happy to work with anyone else
interested.

Elazar
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