[Python-ideas] Generators are iterators

Juancarlo Añez apalala at gmail.com
Wed Dec 10 15:10:11 CET 2014


On Wed, Dec 10, 2014 at 8:06 AM, Oscar Benjamin <oscar.j.benjamin at gmail.com>

> Somehow during the discussion of PEP 479 it was concluded that
> generators will become distinct from iterators. A few people have
> pointed out that this is a harmful distinction (complicates the
> language hard to explain etc). I think it is simply an invalid
> distinction and should be abolished.
> PEP 479 proposes to change the way that generators handle
> StopIteration fall-through by default. However the iterator protocol
> has never formally specified that an iterator should allow
> StopIteration to fall through to a grandparent consumer. I considered
> the fact that StopIteration fall-through was possible to be a
> convenient design feature but it was never a required part of the
> definition of an iterator.
> Many iterators perform other kinds of exception handling without
> allowing that to fall through and many iterators catch StopIteration
> from child iterators without reraising it. This has never lead anyone
> to suggest that such iterators are not true iterators in any way.
> Under PEP 479 generators will handle a StopIteration arising from the
> executing frame differently. The generator will still be an object
> with __iter__ and __next__ that exposes StopIteration at the
> appropriate time to its parent iterator-consumer. In other words it
> will still satisfy the definition of an iterator.
> The PEP says:
> """
> Under this proposal, generators and iterators would be distinct, but
> related, concepts. Like the mixing of text and bytes in Python 2, the
> mixing of generators and iterators has resulted in certain perceived
> conveniences, but proper separation will make bugs more visible.
> """"
> This is just plain false. Under the proposal generators will still be
> iterators. Mixing generators and iterators is nothing like mixing text
> and bytes and never has been. Mixing iterators and iterables is a bit
> like mixing text and the bytes in the sense that it can seem to work
> but then sometimes silently do the wrong thing. Mixing generators and
> iterators is like mixing sets and containers: there is no mixing since
> it is simply a false distinction.
> AFAICT from reading through the discussions this idea has (implicitly)
> followed from the following fallacious argument:
> 1) Bare next creates a problem for generators.
> 2) Therefore we fix it by changing generators.
> 3) Therefore generators are not iterators any more.
> Anyone with experience in propositional logic can see multiple
> fallacies in that argument but actually the biggest mistake is simply
> in the opening premise: bare next is a problem for all iterators.
> Generators are affected precisely because they are iterators.
> Generators were introduced as "a kind of Python iterator, but of an
> especially powerful kind" (PEP 255). The coroutine idea has since
> turned generators into something of a Frankenstein concept but the
> fundamental fact that they are iterators remains unchanged. And
> regardless of how much discussion coroutines generate the fact remains
> that 99% of generators are used purely for iteration.
> I propose to abolish this notion that generators are not iterators and
> to amend the text of the PEP to unambiguously state that generators
> are iterators regardless of any changes to the way they propagate
> StopIteration from the executing frame.
> Oscar
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Juancarlo *Añez*
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