Expose log format to users in assertLogs

[garry-cairns]

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

Linked PRs

• gh-134567: Exposes log format to users in unittest.TestCase.assertLogs #134570
• gh-134567: Move unittest What’s New entry #136630

[vsajip]

@picnixz this appears to be a unittest issue rather than logging? The linked PR doesn't touch logging code.

[picnixz]

Oups my bad! thanks for correctly triaging this

[serhiy-storchaka]

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]

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]

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.

Agreed, the intent here was just to make this feel a little more natural.

[serhiy-storchaka]

@pitrou, what is your opinion about this feature as the original author of assertLogs()?

[pitrou]

That sounds reasonable to me, even if probably a bit niche.