On Mon, Nov 29, 2021, 12:33 AM Paul Bryan <pbr...@anode.ca> wrote: > 1. Noted: Python's for statement will happily iterate over an object that > only implements __next__. > 2. The documentation is pretty clear on what is expected of an iterator: > implement __next__ *and* __iter__. > 3. It is perfectly reasonable for __iter__ to return something other than > self; the documentation already reflects this. > 4. If you believe the documentation is in error or the requirement should > be relaxed, then further discussion is probably warranted. >
Not so much "in error" as "could be clarified." Another couple words like "usually desirable" ... for an iterator to implement .__iter__() would cover it. Maybe I'll make a PR on the docs. I agree that an iterator that doesn't implement .__iter__() is usually unnecessarily a PITA. Especially since "return self" is *usually* a perfectly good implementation. > On Mon, 2021-11-29 at 00:22 -0500, David Mertz, Ph.D. wrote: > > On Mon, Nov 29, 2021, 12:16 AM Paul Bryan <pbr...@anode.ca> wrote: > > And the second link? > > > Same comments, basically. But the more germane thing is that even assuming > a class has both .__next__() and .__iter__(), it is perfectly reasonable > for the latter to return something other than `self`. > > The Foo and Bar classes are slightly contrived, but I've written > production code where e.g. `iter(thing)` returns a new `thing.__class__` > instance rather than self. > > > > On Mon, 2021-11-29 at 00:11 -0500, David Mertz, Ph.D. wrote: If you > > On Sun, Nov 28, 2021, 11:43 PM Paul Bryan <pbr...@anode.ca> wrote: > > According to https://docs.python.org/3/glossary.html#term-iterator and > https://docs.python.org/3/library/stdtypes.html#typeiter, iterators must > implement the __iter__ method. > > > From your first link: > > CPython implementation detail: CPython does not consistently apply the > requirement that an iterator define __iter__(). > > > That said, I don't think the description at the link is very good. > Anyway, it's different from what I teach, and also different from how > Python actually behaves. E.g.: > > >>> class Foo: > ... def __iter__(self): > ... return Bar() > ... > >>> class Bar: > ... def __next__(self): > ... if random() > 0.5: > ... raise StopIteration > ... return "Bar" > ... > >>> for x in Foo(): > ... print(x) > ... > Bar > Bar > Bar > > > Or anyway, what would you call `bar := Bar()` if not "an iterator?! > > On Sun, 2021-11-28 at 22:02 -0500, David Mertz, Ph.D. wrote: > > On Sun, Nov 28, 2021, 8:59 PM Steven D'Aprano > > To be an iterator, your object needs: > > 1. a `__next__` method which returns the next value; > 2. and an `__iter__` method which returns self. > > > That's not quite right. > > An iterator only needs .__next__(), and an iterable only needs > .__iter__(). Returning self is a convenient, and probably the most common, > way of creating an object that is both. But exceptions exist, and remain > iterators and/or iterables. > > > > >
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