Clarify that atexit.unregister matches by equality, not identity

[mdickinson]

BPO	38062
Nosy	@mdickinson, @miss-islington, @iritkatriel, @jdevries3133
PRs	bpo-38062: [doc] clarify that atexit uses equality comparisons internally. #26935 [3.10] bpo-38062: [doc] clarify that atexit uses equality comparisons internally. (GH-26935) #26956 [3.9] bpo-38062: [doc] clarify that atexit uses equality comparisons internally. (GH-26935) #26957

^{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 2021-06-29.18:22:49.469>
created_at = <Date 2019-09-09.08:51:28.823>
labels = ['easy', '3.11', '3.9', '3.10', 'docs']
title = 'Clarify that atexit.unregister matches by equality, not identity'
updated_at = <Date 2021-06-29.18:22:49.468>
user = 'https://github.com/mdickinson'

bugs.python.org fields:

activity = <Date 2021-06-29.18:22:49.468>
actor = 'iritkatriel'
assignee = 'docs@python'
closed = True
closed_date = <Date 2021-06-29.18:22:49.469>
closer = 'iritkatriel'
components = ['Documentation']
creation = <Date 2019-09-09.08:51:28.823>
creator = 'mark.dickinson'
dependencies = []
files = []
hgrepos = []
issue_num = 38062
keywords = ['patch', 'easy']
message_count = 4.0
messages = ['351363', '396748', '396751', '396752']
nosy_count = 5.0
nosy_names = ['mark.dickinson', 'docs@python', 'miss-islington', 'iritkatriel', 'jack__d']
pr_nums = ['26935', '26956', '26957']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue38062'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[mdickinson]

Suppose I have a class that looks like this:

    class A:
        def cleanup(self):
            print("Doing essential cleanup")

and on an instance a = A(), I do: atexit.register(a.cleanup).

Then it's not obvious from the documentation that an atexit.unregister(a.cleanup) will successfully undo the effect of the reigster call: the second a.cleanup is a different object from the first:

    >>> a = A()
    >>> clean1 = a.cleanup
    >>> clean2 = a.cleanup
    >>> clean1 is clean2
    False

Luckily, though the two bound methods are different objects, they're equal:

    >>> clean1 == clean2
    True

and from looking at the source, it's apparent that atexit.unregister compares by equality rather than identity, so everything works.

It would be good to add a sentence to the documentation for atexit.unregister to clarify that this can be expected to work.

[iritkatriel]

New changeset 12803c5 by Jack DeVries in branch 'main':
bpo-38062: [doc] clarify that atexit uses equality comparisons internally. (GH-26935)
12803c5

[iritkatriel]

New changeset 08aa26e by Miss Islington (bot) in branch '3.10':
bpo-38062: [doc] clarify that atexit uses equality comparisons internally. (GH-26935) (GH-26956)
08aa26e

[iritkatriel]

New changeset 02859df by Miss Islington (bot) in branch '3.9':
bpo-38062: [doc] clarify that atexit uses equality comparisons internally. (GH-26935) (GH-26957)
02859df