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1. blueyed

   added

   topic: rewriterelated to the assertion rewrite mechanismrelated to the assertion rewrite mechanism

   type: enhancementnew feature or API change, should be merged into features branchnew feature or API change, should be merged into features branch

   on Jun 23, 2019
2. nedbat commented on Jun 23, 2019

   

   nedbat

   on Jun 23, 2019

   ContributorAuthor

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   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.)
3. blueyed commented on Jun 23, 2019

   

   blueyed

   on Jun 23, 2019

   Contributor

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   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.
4. ikappaki commented on May 12, 2025

   

   ikappaki

   on May 12, 2025

   Last edited by ikappaki

   Contributor

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   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
   

   pytest/src/_pytest/assertion/rewrite.py

   Line 284 in 919ae9d

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

   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:

   pytest/src/_pytest/config/__init__.py

   Lines 1541 to 1574 in 20c51f7

   	def issue_config_time_warning(self, warning: Warning, stacklevel: int) -> None:
   	"""Issue and handle a warning during the "configure" stage.
   	During ``pytest_configure`` we can't capture warnings using the ``catch_warnings_for_item``
   	function because it is not possible to have hook wrappers around ``pytest_configure``.
   	This function is mainly intended for plugins that need to issue warnings during
   	``pytest_configure`` (or similar stages).
   	:param warning: The warning instance.
   	:param stacklevel: stacklevel forwarded to warnings.warn.
   	"""
   	if self.pluginmanager.is_blocked("warnings"):
   	return
   	cmdline_filters = self.known_args_namespace.pythonwarnings or []
   	config_filters = self.getini("filterwarnings")
   	with warnings.catch_warnings(record=True) as records:
   	warnings.simplefilter("always", type(warning))
   	apply_warning_filters(config_filters, cmdline_filters)
   	warnings.warn(warning, stacklevel=stacklevel)
   	if records:
   	frame = sys._getframe(stacklevel - 1)
   	location = frame.f_code.co_filename, frame.f_lineno, frame.f_code.co_name
   	self.hook.pytest_warning_recorded.call_historic(
   	kwargs=dict(
   	warning_message=records[0],
   	when="config",
   	nodeid="",
   	location=location,
   	)
   	)

   • 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
5. luketainton

   added a commit that references this issue on Jan 5, 2026

   chore(deps): update dependency pytest to v9 (#305)

   ...

   4b75154