Is there a way to implement the ** operator on a custom object

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In order for the "splat" operator to work, the type of the object must
populate slot `tp_as_mapping` with a struct of this type:
https://docs.python.org/3/c-api/typeobj.html#c.PyMappingMethods and
have some non-null implementations of the methods this struct is
supposed to contain.

I can do this in C, but I cannot think of a way to do this in Python
proper. Defining all the methods mentioned in PyMappingMethods doesn't
seem to do it. You could try to research this further, and if, indeed
defining all the methods of PyMappingMethods on the Python side
doesn't produce an object that behaves like a proper mapping, you
could probably file a bug report for that.