Another way to disable rewrites?

[nedbat]

I am trying to add a pytest plugin to coverage.py. Because the plugin is in the coverage package, coverage/init.py will be imported when the plugin is imported. Because I'm using coverage to run pytest in the first place, coverage/__init__.py has already been imported. This leads to a warning:

PytestAssertRewriteWarning: Module already imported so cannot be rewritten: coverage

I can disable this warning by adding PYTEST_DONT_REWRITE to the coverage/__init__.py docstring, but then that word is visible in the docstring for my package. Ugly.

Much of pytest is controlled by specially named attributes in modules. It would be great if I could control rewriting by adding an attribute to my module instead of amending the docstring.

If needed, I can provide a reproducer that shows the warning.

[nedbat]

Here is a gist showing the warning happening: https://gist.github.com/nedbat/2ba6834eab6d008dbd1c494c39f61bad (the warning about __main__ is a confusion between coverage's file and pytest's, and only happens with a -e install of coverage, so I am not concerned about it.)

[blueyed]

Yes, controlling this in another way sounds useful.

One solution for this might be to skip the warning during import of the (entrypoint) plugin (and also for -p coverage.pytest_plugin), only explicitly for __init__.py files seen during it (the import process). It should/would still happen for coverage.pytest_plugin itself then.

[ikappaki]

Hi, we've run into a similar issue with Basilisp, where the basilisp package includes a pytest plugin as part of its source tree:

In pyrproject.toml

[tool.poetry.plugins.pytest11]:
basilisp_test_runner = "basilisp.contrib.pytest.testrunner"

The package also provides a CLI that invokes pytest.main(): From src/basilisp/cli.py:

from basilisp import main as basilisp
#...
def test(
    parser: argparse.ArgumentParser,
    args: argparse.Namespace,
    extra: list[str],
) -> None:  # pragma: no cover
#...
        sys.exit(pytest.main(args=list(extra)))

However, since:

1. basilisp declares a pytest plugin, enabling assertion rewriting, and
2. the basilisp module is already imported when pytest.main() is called,

pytest raises the warning:: Module already imported so cannot be rewritten: basilisp

We'd like to suppress this warning for basilisp, as assertion rewriting isn't needed here. While pytest supports suppressing warnings using the -W option:

-W ignore:Module already imported so cannot be rewritten: basilisp:pytest.PytestAssertRewriteWarning:

attempting this like so:

from basilisp import main as basilisp
#...
def test(
    parser: argparse.ArgumentParser,
    args: argparse.Namespace,
    extra: list[str],
) -> None:  # pragma: no cover
#...
      extra = [
            "-W",
            "ignore:Module already imported so cannot be rewritten:* basilisp:pytest.PytestAssertRewriteWarning",
        ] + extra
        sys.exit(pytest.main(args=list(extra)))

fails with the error:

ERROR: while parsing the following warning configuration:
 ignore:Module already imported so cannot be rewritten:* basilisp:pytest.PytestAssertRewriteWarning
 This error occurred:
 Traceback (most recent call last):
  File "d:\src\basilisp\.venv\Lib\site-packages\_pytest\config\__init__.py", line 1918, in parse_warning_filter
    category: type[Warning] = _resolve_warning_category(category_)
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "d:\src\basilisp\.venv\Lib\site-packages\_pytest\config\__init__.py", line 1957, in _resolve_warning_category
    cat = getattr(m, klass)
          ^^^^^^^^^^^^^^^^^
AttributeError: module 'builtins' has no attribute '* basilisp'

this doesn't work because the message contains a literal : (as in ...rewritten: basilisp), which is treated as field separator in -W, corrupting the filter definition.

Since wildcards aren't supported in -W filters one possible solution is to change the warning message in https://github.com/pytest-dev/pytest/blob/919ae9d7c12bec52ea0c18a14299223eadb2b890/src/_pytest/assertion/rewrite.py#L284

to replace : for example with something like ;:

              f"Module already imported so cannot be rewritten; {name}"

So that the -W filter can now work (there is no extra : in the filter any more): -W ignore:Module already imported so cannot be rewritten; basilisp:pytest.PytestAssertRewriteWarning.

Using : in warning messages makes it incompatible with some Python’s warning filter syntax, so replacing it would improve usability. I’m planning to raise a PR to suggest this change.

Alternatives considered

• Use Python's warning.filterwarnings() prior to calling pytest.main:

import warnings
warnings.filterwarnings("ignore", message="Module already imported so cannot be rewritten: basilisp", category=pytest.PytestAssertRewriteWarning)
sys.exit(pytest.main(args=list(extra)))

This doesn't work doesn't work because pytest disables all filters internally when emitting this warning:

https://github.com/pytest-dev/pytest/blob/20c51f70c99ecaa4745d622a69bbe93fbf59ad16/src/_pytest/config/init.py#L1541-L1574

• Users adding explicit filterwarnings in their pyproject.toml:

[tool.pytest.ini_options]
filterwarnings = [
    "ignore:Module already imported so cannot be rewritten. basilisp:pytest.PytestAssertRewriteWarning",
]

This works but requires the user to know and handle this technical limitation explicitly. Ideally, the warning wouldn’t be shown in the first place.

Thanks