contextlib.aclosing()

[belm0]

BPO	40213
Nosy	@ncoghlan, @belm0, @njsmith, @asvetlov, @agronholm, @serhiy-storchaka, @1st1, @corona10, @belm0
PRs	bpo-40213: add contextlib.aclosing() #22640 bpo-41229: Update docs for explicit aclose()-required cases and add contextlib.aclosing() method #21545

^{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 2021-06-11.08:36:18.251>
created_at = <Date 2020-04-07.02:13:32.631>
labels = ['type-feature', 'library', '3.10']
title = 'contextlib.aclosing()'
updated_at = <Date 2021-06-11.08:36:18.250>
user = 'https://github.com/belm0'

bugs.python.org fields:

activity = <Date 2021-06-11.08:36:18.250>
actor = 'John Belmonte'
assignee = 'none'
closed = True
closed_date = <Date 2021-06-11.08:36:18.251>
closer = 'John Belmonte'
components = ['Library (Lib)']
creation = <Date 2020-04-07.02:13:32.631>
creator = 'John Belmonte'
dependencies = []
files = []
hgrepos = []
issue_num = 40213
keywords = ['patch']
message_count = 9.0
messages = ['365885', '365894', '372905', '372908', '372909', '372910', '372911', '377257', '395618']
nosy_count = 10.0
nosy_names = ['ncoghlan', 'jbelmonte', 'njs', 'asvetlov', 'alex.gronholm', 'serhiy.storchaka', 'yselivanov', 'nikicat', 'corona10', 'John Belmonte']
pr_nums = ['22640', '21545']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40213'
versions = ['Python 3.10']

[belm0]

Please add aclosing() to contextlib, the async equivalent of closing().

It's needed to ensure deterministic call of aclose() on the resource object at block exit.

It's been available in the async_generator module for some time. However that module is intended to provide async generators to Python 3.5, so it's odd for apps using modern Python versions to depend on it only for aclosing().

https://github.com/python-trio/async_generator/blob/22eddc191c2ae3fc152ca13cf2d6fa55ac3f1568/async_generator/_util.py#L6

[agronholm]

Seconded.

[serhiy-storchaka]

Do you have use cases or want to add it for pure "symmetry" reasons?

contextlib.closing() was added when context managers were new, and many classes that own resources did have the close() method but did not support the context manager protocol. Now most of these classes are context managers, and new classes usually are written with the support of the context manager protocol from beginning. The usefulness of contextlib.closing() is much smaller now.

[agronholm]

They are both still useful, particularly with third party libraries. Just yesterday I found myself looking for aclosing() in contextlib, only to remember this issue.

[belm0]

Given the lack of deterministic cleanup for iterators (https://www.python.org/dev/peps/pep-0533/), aclosing() is the way to ensure deterministic cleanup given any API using async iteration.

[belm0]

highlighting from PEP-533:

Async generators cannot do cleanup at all without some mechanism for deterministic cleanup that people will actually use, and async generators are particularly likely to hold resources like file descriptors.

[agronholm]

I think the most important use case for these is closing async generators deterministically, since they don't support the async context manager protocol.

[corona10]

see https://bugs.python.org/issue41229

[belm0]

merged for Python 3.10