Due to shared code between the 2 and 3 forms of pow, the following code causes a TypeError:

class A:
    def __init__(self, val):
        self.val = val
    
    def __ipow__(self, other):
        return NotImplemented

class B:
    def __init__(self, val):
        self.val = val
    
    def __rpow__(self, other):
        return A(other.val ** self.val)

a = A(2)
b = B(2)
a **= b


(https://stackoverflow.com/questions/58141475/ipow-raising-typeerror-when-left-hand-side-object-returns-notimplemented)

Meant to say "... 2 and 3 arguments forms of pow, ..."

The document says(https://docs.python.org/3.9/reference/datamodel.html?highlight=__rpow__#object.__rpow__)：
>Note that ternary pow() will not try calling __rpow__() (the coercion rules would become too complicated).

This isn't the ternary form of pow(), the documentation there is referring to the `pow(base, exp, modulus)`.

Oh, I see. Thank you.

It turns out **= ONLY calls __ipow__ and neither __pow__ or  __rpow__ as the data model says should be called.

- Data Model: https://docs.python.org/3/reference/datamodel.html#object.__ipow__
- PyNumber_InPlacePower(): https://github.com/python/cpython/blob/802726acf6048338394a6a4750835c2cdd6a947b/Objects/abstract.c#L1159
- ternary_op (which is what is used to implement PyNumber_InPlacePower(): https://github.com/python/cpython/blob/802726acf6048338394a6a4750835c2cdd6a947b/Objects/abstract.c#L849

I have opened https://bugs.python.org/issue41688 to track the documentation fixes so that this can become a release blocker for Python 3.10.