Guido van Rossum wrote:
> On Tue, Sep 14, 2021 at 3:49 PM Brandt Bucher brandtbuc...@gmail.com
> wrote:
> > I think it's also worth noting that a missing "`__iter__` that returns
> > self" is trivial to recover from... just use a new reference to the
> > iterator instead. The overhead of a method call for this convention almost
> > seems silly.
> The use case is this:
Yeah, I understand that. But what I'm hinting that is that the `GET_ITER`
opcode and `iter` builtin *could* gracefully handle this situation when called
on something that doesn't define `__iter__` but does define `__next__`.
Pseudocode:
def iter(o):
if hasattr(o, "__iter__"):
return o.__iter__()
elif hasattr(o, "__next__"):
# Oh well, o.__iter__() would have just returned o anyways...
return o
raise TypeError
This would be implemented at the lowest possible level, in `PyObject_GetIter`.
> > What worries me most about changing the current "requirement" is that it
> > may create either confusion or backward compatibility issues for
> > `collections.abc.Iterator` (which is a subtype of `Iterable`, and thus
> > requires `__iter__`).
> If you explicitly inherit from Iterator, you inherit a default
> implementation of __iter__ (that returns self, of course). If you merely
> register, it's up to you to comply. And sometimes people register things
> that don't follow the letter of the protocol, just to get things going.
> (This is common for complex protocols like Mapping, where some function you
> have no control over insists on a Mapping but only calls one or two common
> methods.
Yeah, I was thinking about cases like `isinstance(o, Iterator)`, where `o`
defines `__iter__` but not `__next__`. Even though this code might start
returning the "right" answer, it's still a backward-compatibility break. Not
sure what the true severity would be, though...
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