[Python-ideas] __iter__(), keys(), and the mapping protocol

[Elias Tarhini]

This has been bouncing around in my head for a while regarding the
requisite keys() method on mappings:

How come the ** unpacking operator, a built-in language feature, relies on
a non-dunder to operate?

To me, I mean to say, requiring that classes implement keys() – a method
whose name is totally undistinguished – in order to conform to the mapping
protocol feels like a design running counter to Python's norm of using
dunders for everything "hidden". I am not sure if it feels dirty to anybody
else, however. Interestingly, the docs already say
<https://docs.python.org/3/reference/datamodel.html#object.__iter__>
that *[f]or
mappings, [__iter__()] should iterate over the keys of the container*, but
it of course is not enforced in any way at present.

So, then — how about enforcing it? Should __iter__(), for the reasons
above, replace the current purpose of keys() in mappings?

I'm not properly equipped at the moment to mess around with CPython
(sorry), but I assume at a minimum this would entail either replacing all
instances of PyMapping_Keys() with PyObject_GetIter() or alternatively
changing  PyMapping_Keys() to call the latter.

Does it sound like a reasonable change overall?

Eli
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