On Tue, Sep 14, 2021 at 9:03 PM Steven D'Aprano <st...@pearwood.info> wrote:
> On Tue, Sep 14, 2021 at 12:33:32PM -0700, Guido van Rossum wrote: > > My view of this is: > > > > A. It's not an iterator if it doesn't define `__next__`. > > > > B. It is strongly recommended that iterators also define `__iter__`. > > > > In "standards" language, I think (A) is MUST and (B) is merely OUGHT or > > maybe SHOULD. > > That's not what the docs say :-) > > https://docs.python.org/3/library/stdtypes.html#iterator-types > Huh, so it does. And in very clear words as well. I still don't think this should be enforced by checks for the presence of __iter__ in situations where it's not going to be called (e.g. in iter() itself and in "for x in it"). But since this is a longstanding convention and matches collections.abc.Iterator (and typing.Iterator) we might as well *document* it to be the case. > Part of the problem is that there are two kinds of thing that we call > "iterator": > > 1. Objects that we implicitly or explicitly pass to `iter()` in order to > return an interator object; they only need to define an `__iter__` > method that returns the actual iterator object itself. > No, we don't call that an iterator. That's an *Iterable*. In this the docs you point to are actually weak: - It doesn't use the term Iterable at all but describe it as "container objects" or "containers". - It says " Sequences, described below in more detail, always support the iteration methods." That's wrong, or at the very least misleading, since a sequence itself *only* supports __iter__ -- it's the Iterator returned by s.__iter__() that supports __next__. > (That's a slight simplification, because iter() will fall back on the > Sequence Protocol if `__iter__` isn't defined. But to my mind, that > makes Sequence Protocol objects *iterables* not iterators.) > Right, it's wrong. > 2. Iterator objects themselves, which are defined by a protocol, not a > type. The iterator object MUST define both `__iter__` and `__next__`, > and the `__iter__` method MUST return self. > So you say. I will compromise and agree that Iterators MUST have __next__ and SHOULD have __iter__ returning self. The distinction is that without __next__ it's not an Iterator. But without __iter__ it's merely a broken Iterator (that nevertheless works in most situations). -- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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