There was just a bpo on this — discussion of exactly what an Iterable th or is.
I’m pretty sure the conclusion was that in order to be an Iterator, an object needs to have __iter_. I’m not totally sure that it has to return self, but that is the expected behavior, so that it can be re-entrant. Of course, Python being Python, if in a given use case, you don’t need __iter__, you don’t have to have it. Just like the fact that you can pass anything with an appropriate read() method into json.load() will work, but that doesn’t make it a File. -CHB On Sun, Nov 28, 2021 at 9:42 PM David Mertz, Ph.D. <david.me...@gmail.com> wrote: > 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. >> >> >> >> >> _______________________________________________ > Python-ideas mailing list -- python-ideas@python.org > To unsubscribe send an email to python-ideas-le...@python.org > https://mail.python.org/mailman3/lists/python-ideas.python.org/ > Message archived at > https://mail.python.org/archives/list/python-ideas@python.org/message/WZNQ3BSWI2VYFKLEUZ4QNGZLH65PESYD/ > Code of Conduct: http://python.org/psf/codeofconduct/ > -- Christopher Barker, PhD (Chris) Python Language Consulting - Teaching - Scientific Software Development - Desktop GUI and Web Development - wxPython, numpy, scipy, Cython
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