[Python-ideas] Generators are iterators

Steven D'Aprano steve at pearwood.info
Wed Dec 10 15:18:59 CET 2014

On Wed, Dec 10, 2014 at 12:36:59PM +0000, Oscar Benjamin wrote:

> 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.

I certainly hope so.

Currently, generators are iterators: they obey the iterator protocol and 
the Iterator ABC correctly registers them as instances.

py> def gen():
...     yield 1
py> it = gen()
py> iter(it) is it  # Iterator protocol is obeyed.
py> hasattr(it, '__next__')
py> from collections.abc import Iterator
py> isinstance(it, Iterator)  # And registered with the ABC.

Surely this isn't going to change? If it does, I expect that's going to 
break an enormous amount of code.

If generators are to cease to be iterators, what will they be?

> 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.

Sounds reasonable to me.


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