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

[Oscar Benjamin]

On 15 July 2013 18:39, Guido van Rossum <guido at python.org> wrote:
> On Mon, Jul 15, 2013 at 3:40 AM, Oscar Benjamin
> <oscar.j.benjamin at gmail.com> wrote:
>> I would like it if the
>> unpacking syntax could somehow be used for iterators. For example:
>>
>>     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. Using the tuple syntax is
>> probably confusing but it would be great if there were some way to
>> spell this and get an iterator instead of a concrete collection.
>
> I think this is going down a slippery slope that could jeopardize the
> whole PEP, which is nice and non-controversial so far. The problem is
> that "tuples" (more precisely, things separated by commas) are already
> overloaded to the point where both the parser and most human readers
> are strained to the max to tell the different cases apart. For
> example, this definitely creates a tuple:
>
> a = 1, 2, 3
>
> Now consider this:
>
> b = 2, 3
> a = 1, *b
>
> Why would that not create the same tuple?

Yeah, this is what I mean that tuple syntax is confusing. I think,
though, that it would be good if some way of creating iterators
evolved from this.

Consider that while list, set and dict comprehensions can create
lists, sets and dicts. Generator expressions can create tuples,
OrderedDicts, blists, deques and many more. They can also be used with
folds like min, max, sum, any, all and many more. Creating an iterator
provides a much more general tool then creating a particular concrete
type.


Oscar