Add an assertNoLogs context manager to unittest TestCase

[kitchoi]

BPO	39385
Nosy	@vsajip, @mdickinson, @rbtcollins, @ezio-melotti, @voidspace, @kitchoi, @francoisfreitag, @remilapeyre, @tirkarthi
PRs	bpo-39385: Add an assertNoLogs context manager to unittest.TestCase (GH-18067) #18067 bpo-39385: Add missing what's new entry for TestCase.assertNoLogs #22317

^{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 2020-07-01.21:09:42.076>
created_at = <Date 2020-01-18.20:25:23.335>
labels = ['type-feature', 'library', '3.10']
title = 'Add an assertNoLogs context manager to unittest TestCase'
updated_at = <Date 2020-09-19.09:32:42.441>
user = 'https://github.com/kitchoi'

bugs.python.org fields:

activity = <Date 2020-09-19.09:32:42.441>
actor = 'mark.dickinson'
assignee = 'none'
closed = True
closed_date = <Date 2020-07-01.21:09:42.076>
closer = 'vinay.sajip'
components = ['Library (Lib)']
creation = <Date 2020-01-18.20:25:23.335>
creator = 'Kit Yan Choi'
dependencies = []
files = []
hgrepos = []
issue_num = 39385
keywords = ['patch']
message_count = 4.0
messages = ['360250', '367129', '367134', '372794']
nosy_count = 9.0
nosy_names = ['vinay.sajip', 'mark.dickinson', 'rbcollins', 'ezio.melotti', 'michael.foord', 'Kit Yan Choi', 'francois.freitag', 'remi.lapeyre', 'xtreak']
pr_nums = ['18067', '22317']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue39385'
versions = ['Python 3.10']

[kitchoi]

assertLogs is really useful (bpo-18937). Unfortunately it does not cover the use cases where one wants to ensure no logs are emitted.

Similar to assertLogs, we can have a context manager for asserting no logs, something like this?:

with assertNoLogs(logger, level):
    ...

If logs are unexpected found, the test would fail with the logs captured included in the error message.

Happy to submit a PR if there is interest.

[remilapeyre]

This makes sense, should assertNoWarns() be added too?

[kitchoi]

Thank you for looking into this.
Yes, I agree it makes sense to have assertNoWarns for the same reason.

[vsajip]

New changeset 6b34d7b by Kit Choi in branch 'master':
bpo-39385: Add an assertNoLogs context manager to unittest.TestCase (GH-18067)
6b34d7b