[Python-Dev] Tricky way of of creating a generator via a comprehension expression

[Nick Coghlan]

On 25 November 2017 at 15:27, Nathaniel Smith <njs at pobox.com> wrote:
> On Fri, Nov 24, 2017 at 9:04 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>>     def example():
>>         comp1 = yield from [(yield x) for x in ('1st', '2nd')]
>>         comp2 = yield from [(yield x) for x in ('3rd', '4th')]
>>         return comp1, comp2
>
> Isn't this a really confusing way of writing
>
> def example():
>     return [(yield '1st'), (yield '2nd')], [(yield '3rd'), (yield '4th')]

A real use case wouldn't be iterating over hardcoded tuples in the
comprehensions, it would be something more like:

    def example(iterable1, iterable2):
        comp1 = yield from [(yield x) for x in iterable1]
        comp2 = yield from [(yield x) for x in iterable2]
        return comp1, comp2

Defining an interesting for loop isn't the point of the example though
- it's just to show that if you're inside a generator, you can already
make a subgenerator comprehension do something sensible by sticking
"yield from" in front of it (and have actually been able to do so
since 3.3, when "yield from" was first introduced).

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia