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.

Friendly reminder that this issue is currently blocking the 3.10a5 release. If you are ok with waiting for the next release to include the fix, please say so here or drop me an email/

I'm totally fine with pushing this until b1 since this has been broken for  ages.

New changeset cc02b4f2e810ab524d845daa18bc94df5b092dd8 by Alex in branch 'master':
bpo-38302: __pow__/__rpow__ now called when __ipow__ returns NotImplemented (#16459)
https://github.com/python/cpython/commit/cc02b4f2e810ab524d845daa18bc94df5b092dd8

messages: + msg376208
versions: - Python 3.8, Python 3.9

messages: + msg375825

versions: + Python 3.8, Python 3.9, Python 3.10, - Python 3.6, Python 3.7