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Expose log format to users in assertLogs #134567
Description
Feature or enhancement
Proposal:
Use-case
Someone works in an organisation with a custom log formatter. They want to make assertions about logs in a particular test and would like the assertions to match the format used in the org. Currently the format is static in assertLogs.
Example usage
format = "[No.1: the larch] %(levelname)s:%(name)s:%(message)s"
formatter = logging.Formatter(format)
with self.assertLogs(formatter=formatter) as cm:
log_foo.info("1")
log_foobar.debug("2")
self.assertEqual(cm.output, ["[No.1: the larch] INFO:foo:1"])Proposed change
My branch already has my proposed change and tests are passing. Since this would be my first contribution if accepted I'm raising the issue since I don't yet have a feel for what constitutes a "minor" enhancement that doesn't need an issue ref per the developer guide. Having raised this I'll create a PR as per the developer guide.
Has this already been discussed elsewhere?
This is a minor feature, which does not need previous discussion elsewhere
Links to previous discussion of this feature:
No response
Activity
vsajip commented on May 26, 2025
@picnixz this appears to be a unittest issue rather than logging? The linked PR doesn't touch logging code.
picnixz commented on May 26, 2025
Oups my bad! thanks for correctly triaging this
serhiy-storchaka commented on Jun 23, 2025
You can change the formatter by executing logging.getLogger().handlers[0].setFormatter(formatter) in the with block. This is just a little verbose. You need to change the code if you also specified a logger.
@vsajip, what is your opinion about usefulness of such feature?
vsajip commented on Jun 23, 2025
It seems like a "Wouldn't it be nice if ... ?" feature rather than an essential one, and in any case it seems like a change in unittest rather than logging. Your suggestion would be using internal logging API (handlers attribute of Logger isn't a documented API), so I wouldn't recommend it to be done in user code. The documented purpose of assertLogs relates to assertions about loggers and levels only, so changing it to encompass formats seems out of scope and not necessary. However, unittest maintainers might have a different view.
garry-cairns commented on Jun 23, 2025
Agreed, the intent here was just to make this feel a little more natural.
serhiy-storchaka commented on Jun 24, 2025
@pitrou, what is your opinion about this feature as the original author of assertLogs()?
pitrou commented on Jun 25, 2025
That sounds reasonable to me, even if probably a bit niche.
Addresses review comments on pythongh-134567
serhiy-storchaka commented on Jul 2, 2025
Thank you for your contribution @garry-cairns.
gh-134567: Add the formatter parameter in unittest.TestCase.assertLogs (
pythongh-134567: Add the formatter parameter in unittest.TestCase.ass…
pythongh-134567: Add the formatter parameter in unittest.TestCase.ass…
pythongh-134567: Add the formatter parameter in unittest.TestCase.ass…
gh-134567: Move unittest What’s New entry (#136630)
pythongh-134567: Add the formatter parameter in unittest.TestCase.ass…
pythongh-134567: Move unittest What’s New entry (python#136630)
pythongh-134567: Add the formatter parameter in unittest.TestCase.ass…
pythongh-134567: Move unittest What’s New entry (python#136630)