[Python-Dev] Tricky way of of creating a generator via a comprehension expression (original) (raw)
Nick Coghlan ncoghlan at gmail.com
Sat Nov 25 02:10:12 EST 2017
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On 25 November 2017 at 16:18, Nathaniel Smith <njs at pobox.com> wrote:
On Fri, Nov 24, 2017 at 9:39 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
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 Do you have a real use case? This seems incredibly niche...
That's not how backwards compatibility works - we were suggesting getting rid of this syntax, because there was no current way to make it do anything sensible.
It turns out there is a way to make it behave reasonably - you just need to stick "yield from" in front of it, and it goes back to being equivalent to the corresponding for loop (the same as the synchronous version).
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 I submit that this would still be easier to understand if written out like: def mapiterabletoyieldvalues(iterable): "Yield the values in iterable, then return a list of the values sent back." result = [] for obj in iterable: result.append(yield obj) return result def example(iterable1, iterable2): values1 = yield from mapiterabletoyieldvalues(iterable1) values2 = yield from mapiterabletoyieldvalues(iterable2) return values1, values2
The same can be said for comprehensions in general. Composing them with coroutines certainly doesn't make either easier to understand, but I don't think replacing the comprehension with its full imperative form is particularly helpful in aiding that understanding.
Cheers, Nick.
-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia
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