PyNumber_InPlacePower ignores o3 if o1 implements __ipow__

[hodgestar]

BPO	41903
Nosy	@brettcannon, @ammaraskar

^{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 = None
created_at = <Date 2020-10-01.15:10:02.476>
labels = ['type-bug', '3.8', '3.9', '3.10', 'expert-C-API', '3.7']
title = 'PyNumber_InPlacePower ignores o3 if o1 implements __ipow__'
updated_at = <Date 2020-10-01.15:48:52.092>
user = 'https://bugs.python.org/hodgestar'

bugs.python.org fields:

activity = <Date 2020-10-01.15:48:52.092>
actor = 'ammar2'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['C API']
creation = <Date 2020-10-01.15:10:02.476>
creator = 'hodgestar'
dependencies = []
files = []
hgrepos = []
issue_num = 41903
keywords = []
message_count = 5.0
messages = ['377758', '377760', '377761', '377766', '377767']
nosy_count = 3.0
nosy_names = ['brett.cannon', 'hodgestar', 'ammar2']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41903'
versions = ['Python 3.5', 'Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

Linked PRs

• gh-86069: correct object.__ipow__() signature #130103
• gh-86069: Add more PyNumber_InPlacePower() tests #130111
• [3.13] gh-86069: Add more PyNumber_InPlacePower() tests (GH-130111) #130211
• [3.12] gh-86069: Add more PyNumber_InPlacePower() tests (GH-130111) #130212
• gh-86069: Do not ignore the third argument in slot_nb_inplace_power() #130225

[hodgestar]

The documentation for PyNumber_InPlacePower [1] reads:

This is the equivalent of the Python statement o1 **= o2 when o3 is Py_None, or an in-place variant of pow(o1, o2, o3) otherwise. If o3 is to be ignored, pass Py_None in its place (passing NULL for o3 would cause an illegal memory access).

However, if a class A implements __ipow__ then PyNumber_InPlacePower(o1, o2, o3) ALWAYS ignores o3 if o1 is an instance of A.

This happens because if A implements __ipow__ then PyNumber_InPlacePower always calls the nb_inplace_power slot [2] and the slot always drops the third argument [3].

This appears to have always been the case in Python, so likely a small documentation patch is all that is required. If people agree, I will open a documentation pull request.

[1] https://docs.python.org/3/c-api/number.html?highlight=pynumber_inplacepower#c.PyNumber_InPlacePower

[2] https://github.com/python/cpython/blob/master/Objects/abstract.c#L1164

[3] https://github.com/python/cpython/blob/master/Objects/typeobject.c#L6631-L6636

[hodgestar]

The documentation for __ipow__ [4] suggests that the intention was to support the modulus argument, so perhaps that argues for fixing the behaviour of PyNumber_InPlacePower.

[4] https://docs.python.org/3/reference/datamodel.html#object.\_\_ipow__

[ammaraskar]

Huh, this is weird. I wonder why the data model provides the signature

  object.__ipow__(self, other[, modulo])

if the slot always seems to drop the third argument.

Adding Brett to the nosy who found another issue around the special casing of pow/** recently. See also bpo-38302