[3.10] __pow__ and __rpow__ are not reached when __ipow__ returns NotImplemented for **=

[DeepSpace]

BPO	38302
Nosy	@brettcannon, @serhiy-storchaka, @ammaraskar, @hongweipeng, @pablogsal
PRs	bpo-38302. __rpow__ now called when __ipow__ returns NotImplemented #16459 bpo-38302: when __ipow__ returns NotImplemented, fall back to __pow__/__rpow__ #24587

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-02-26.19:59:01.203>
created_at = <Date 2019-09-27.21:55:15.670>
labels = ['interpreter-core', 'type-bug', '3.10', 'release-blocker']
title = '[3.10] __pow__ and __rpow__ are not reached when __ipow__ returns NotImplemented for **='
updated_at = <Date 2021-02-26.19:59:01.202>
user = 'https://bugs.python.org/DeepSpace'

bugs.python.org fields:

activity = <Date 2021-02-26.19:59:01.202>
actor = 'brett.cannon'
assignee = 'none'
closed = True
closed_date = <Date 2021-02-26.19:59:01.203>
closer = 'brett.cannon'
components = ['Interpreter Core']
creation = <Date 2019-09-27.21:55:15.670>
creator = 'DeepSpace'
dependencies = []
files = []
hgrepos = []
issue_num = 38302
keywords = ['patch']
message_count = 10.0
messages = ['353419', '353422', '353503', '353505', '353595', '375825', '376208', '386108', '386124', '387741']
nosy_count = 6.0
nosy_names = ['brett.cannon', 'serhiy.storchaka', 'ammar2', 'hongweipeng', 'pablogsal', 'DeepSpace']
pr_nums = ['16459', '24587']
priority = 'release blocker'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38302'
versions = ['Python 3.10']

[DeepSpace]

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)

[DeepSpace]

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

[hongweipeng]

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).

[ammaraskar]

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

[hongweipeng]

Oh, I see. Thank you.

[brettcannon]

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():

  cpython/Objects/abstract.c

  Line 1159 in 802726a

  PyObject *

• ternary_op (which is what is used to implement PyNumber_InPlacePower():

  cpython/Objects/abstract.c

  Line 849 in 802726a

  /*

[brettcannon]

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.

[pablogsal]

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/

[brettcannon]

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

[brettcannon]

New changeset cc02b4f by Alex in branch 'master':
bpo-38302: __pow__/rpow now called when __ipow__ returns NotImplemented (bpo-16459)
cc02b4f