Make PyNumber_Index() always returning an exact int instance

[serhiy-storchaka]

BPO	40792
Nosy	@warsaw, @mdickinson, @serhiy-storchaka
PRs	bpo-40792: Make PyNumber_Index() always returning exact int. #20443

^{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 2020-05-28.07:36:23.218>
created_at = <Date 2020-05-27.07:57:47.257>
labels = ['interpreter-core', 'type-feature', '3.10']
title = 'Make PyNumber_Index() always returning an exact int instance'
updated_at = <Date 2020-05-28.07:36:23.217>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2020-05-28.07:36:23.217>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2020-05-28.07:36:23.218>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2020-05-27.07:57:47.257>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 40792
keywords = ['patch']
message_count = 4.0
messages = ['370054', '370064', '370071', '370168']
nosy_count = 3.0
nosy_names = ['barry', 'mark.dickinson', 'serhiy.storchaka']
pr_nums = ['20443']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40792'
versions = ['Python 3.10']

[serhiy-storchaka]

Currently PyNumber_Index() can return something that's an instance of a strict subclass of int. For example PyNumber_Index(Py_True) returns Py_True. The same for operator.index():

>>> import operator
>>> operator.index(True)
True

The proposed PR makes it always return an int.

To avoid possible overhead for creating temporary integer object, added private function _PyNumber_Index() with the past behavior. It can be used for short-living integer objects which for which only its value will be used, but not its methods. For example in the implementation of PyLong_AsLong() and similar functions.

See also bpo-17576.

[serhiy-storchaka]

This change conflicts with the test added in bpo-26202. We perhaps should revert bpo-26202. This can also affect the user code which depends on range() attributes start, stop and step being instances of the int subclass.

[mdickinson]

The behaviour change for range sounds reasonable to me.

Just to make sure I understand the impact of the change:

• For Python < 3.10, we sometimes convert the range inputs to Python ints, and sometimes don't. For example, a start value of np.int64(5) would be converted, but a value of True would not be.

• With this change, we'd always convert a non-int to an int, so both np.int64(1) and True would be converted to a 1 of exact type int.

IMO this is fine; the new behaviour seems more consistent than the old.

>>> import numpy as np
>>> start = np.int64(2)
>>> range(start, 5).start is start
False
>>> start = True
>>> range(start, 5).start is start
True

[serhiy-storchaka]

New changeset 5f4b229 by Serhiy Storchaka in branch 'master':
bpo-40792: Make the result of PyNumber_Index() always having exact type int. (GH-20443)
5f4b229