gh-86069: correct object.__ipow__() signature

[skirpichev]

There is no way to call ternary form of this dunder.

• Issue: PyNumber_InPlacePower ignores o3 if o1 implements __ipow__ #86069

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[skirpichev]

See

cpython/Objects/typeobject.c

Lines 9905 to 9911 in 28f5e3d

	/* Can't use SLOT1 here, because nb_inplace_power is ternary */
	static PyObject *
	slot_nb_inplace_power(PyObject *self, PyObject * arg1, PyObject *arg2)
	{
	PyObject *stack[2] = {self, arg1};
	return vectorcall_method(&_Py_ID(__ipow__), stack, 2);
	}

So, PyNumber_InPlacePower just ignores the o3 now. Maybe its docs also should be adjusted?

[auvipy]

this seems to be correct change.

[serhiy-storchaka]

Look at wrap_ternaryfunc(). The __ipow__ wrapper accepts 1 or 2 positional arguments (not including self). An extension type can implement slot_nb_inplace_power that does not ignore the third argument.

[skirpichev]

An extension type can implement slot_nb_inplace_power that does not ignore the third argument.

How you can invoke it? (excluding a direct call of the dunder)

[serhiy-storchaka]

By a direct call of __ipow__().

[skirpichev]

"excluding a direct call of the dunder"?)

[serhiy-storchaka]

Don't exclude it. Signature describes the direct call.

[skirpichev]

Well, why someone will prefer a dunder method over a public method, that does same?)

Edit: similar to #122260 (3-arg __rpow__() - also expected to be directly called).