CLI -W warning filters appear to escape module regexes — intended behavior?

[j-c-c]

I’m trying to understand pytest’s warning filter behavior for CLI -W options vs filterwarnings in config.

This works in tox.ini:

[pytest]
filterwarnings =
    error
    ignore::UserWarning:tests.*

But the equivalent CLI form does not seem to work:

python -m pytest -W error -W "ignore::UserWarning:tests.*"

while this DOES work:

python -m pytest -W error -W "ignore::UserWarning:tests.test_utils"

Looking at _pytest/config/__init__.py, it appears CLI -W filters are parsed with escape=True:

if module and escape:
    module = re.escape(module) + r"\Z"

which seems to force exact module matching for CLI filters.

Am I understanding correctly that wildcard/regex module matching is intentionally unsupported for CLI -W, and only supported in config-file filterwarnings?

[sueun-dev]

yeah, you're reading it right. This is basically the same split as #8759 (comment)

filterwarnings keeps regex-style matching, but CLI -W / --pythonwarnings goes through the escaped path. Current main still has config filters calling parse_warning_filter(..., escape=False) and CLI filters calling it with escape=True; for the module field that becomes re.escape(module) + r"\\Z", so tests.* won't match tests.test_utils.

For CLI you'd need the concrete module name, or keep the package-wide regex case in config.

[siinn1706]

There is also a way to pass the regex through the command line: override the filterwarnings ini option with -o, rather than using -W.

For Bash:

python -m pytest -o $'filterwarnings=error\nignore::UserWarning:^tests\\.'

This sets two config filters: turn warnings into errors, then ignore UserWarning from modules starting with tests.. It replaces the configured filterwarnings list for this invocation. Do not also pass -W error: CLI warning filters take precedence over config filters.

The distinction you found is intentional: -W treats the module field as a literal full module name, whereas filterwarnings treats it as a regex matched at the beginning of the module name. So tests.* in -W is not a wildcard, and Python's -W module field is not a substring match either.

One correction to the earlier workaround: -p no:warnings disables pytest's warnings plugin, so it should not be combined with filterwarnings as a way to apply those pytest filters.

I checked on Python 3.12.10 / pytest 9.1.1 with a warning emitted in tests.test_warning: the wildcard -W case failed under -W error, the exact-module -W case passed, and the two-line -o filterwarnings=... case passed.

References: warning filter syntax and precedence, overriding config options.

Prepared with AI assistance; the behavior above was checked locally.