[Python-ideas] PEP for issue2292, "Missing *-unpacking generalizations"

[Jan Kaliszewski]

15.07.2013 12:40, Oscar Benjamin wrote:

>     first_line = next(inputfile)
>     # inspect first_line
>     for line in chain([first_line], inputfile):
>         # process line
>
> could be rewritten as
>
>     first_line = next(inputfile):
>     for line in first_line, *inputfile:
>         pass
>
> without reading the whole file into memory.

Please note, that with PEP 448 syntax you could express it by:

     first_line = next(inputfile)
     for line in (*it for it in ([first_line], inputfile)):
         ...

Event now, in Python 3.3, you can[*] write:

     first_line = next(inputfile)
     for line in [(yield from it) for it in [[first_line], inputfile]]:
         ...

Cheers,
*j


[*] Please note that it is `yield from` within a *list comprehension*,
not a generator expression... And that this list cimprehension still
evaluates to a *generator*, not a list!  (a [None, None] list is set
as StopIteration's value when the generator is exhausted)

An interesting fact (but understandable after a though) is that:

while a generator created with:
     [(yield from it) for it in [[1,2,3], 'abc']]
produces items:
     1, 2, 3, 'a', 'b', 'c',

a generator created with:
     ((yield from it) for it in [[1,2,3], 'abc'])
produces items:
     1, 2, 3, None, 'a', 'b', 'c', None

I am not sure if ability to use it that way is only an implementation
artifact, but it works.