[Python-Dev] Why is nb_inplace_power ternary?

["Martin v. Löwis"]

#1653736 reports that slot_nb_inplace_power has the wrong
type: it should be a ternary function, but only is a binary.
The proposed change is to make it ternary, and to invoke __ipow__
with three arguments.

In researching this, I came to wonder why nb_inplace_power is
ternary in the first place. It is the implementation of

foo **= bar

(and that's its only use), so it ought to be binary. Historically,
the reason probably is that it was copied from nb_power, which is
ternary because it also implements pow()).

So here is my proposed change:

1. For 2.5.1, rewrite slot_nb_inplace_power to raise an exception
    if the third argument is not None, and then invoke __ipow__
    with only one argument.

2. For 2.6, change the API to make nb_inplace_power binary.

Optionally, in 2.5, the exception could be added into other places
as well, e.g. PyNumber_InPlacePower and instance_ipow (rather than
invoking __ipow__ with a third argument if its not None).

Comments?

Regards,
Martin