Fix `assert mod not in mods` crash

Fix #27806.

Co-authored-by: Loïc Estève <loic.esteve@ymail.com>
Co-authored-by: Ran Benita <ran@unusedvar.com>
Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>

changelog/9765.bugfix.rst

@@ -0,0 +1,3 @@
+Fixed a frustrating bug that afflicted some users with the only error being ``assert mod not in mods``. The issue was caused by the fact that ``str(Path(mod))`` and ``mod.__file__`` don't necessarily produce the same string, and was being erroneously used interchangably in some places in the code.
+
+This fix also broke the internal API of ``PytestPluginManager.consider_conftest`` by introducing a new parameter -- we mention this in case it is being used by external code, even if marked as *private*.

src/_pytest/config/__init__.py

@@ -634,7 +634,8 @@ class PytestPluginManager(PluginManager):
     def _importconftest(
         self, conftestpath: Path, importmode: Union[str, ImportMode], rootpath: Path
     ) -> types.ModuleType:
-        existing = self.get_plugin(str(conftestpath))
+        conftestpath_plugin_name = str(conftestpath)
+        existing = self.get_plugin(conftestpath_plugin_name)
         if existing is not None:
             return cast(types.ModuleType, existing)
 
@@ -666,7 +667,7 @@ class PytestPluginManager(PluginManager):
                     assert mod not in mods
                     mods.append(mod)
         self.trace(f"loading conftestmodule {mod!r}")
-        self.consider_conftest(mod)
+        self.consider_conftest(mod, registration_name=conftestpath_plugin_name)
         return mod
 
     def _check_non_top_pytest_plugins(
@@ -746,9 +747,11 @@ class PytestPluginManager(PluginManager):
                     del self._name2plugin["pytest_" + name]
             self.import_plugin(arg, consider_entry_points=True)
 
-    def consider_conftest(self, conftestmodule: types.ModuleType) -> None:
+    def consider_conftest(
+        self, conftestmodule: types.ModuleType, registration_name: str
+    ) -> None:
         """:meta private:"""
-        self.register(conftestmodule, name=conftestmodule.__file__)
+        self.register(conftestmodule, name=registration_name)
 
     def consider_env(self) -> None:
         """:meta private:"""

testing/acceptance_test.py

@@ -1,6 +1,7 @@
 import dataclasses
 import importlib.metadata
 import os
+import subprocess
 import sys
 import types
 
@@ -1390,3 +1391,61 @@ def test_doctest_and_normal_imports_with_importlib(pytester: Pytester) -> None:
     )
     result = pytester.runpytest_subprocess()
     result.stdout.fnmatch_lines("*1 passed*")
+
+
+@pytest.mark.skip(reason="Test is not isolated")
+def test_issue_9765(pytester: Pytester) -> None:
+    """Reproducer for issue #9765 on Windows
+
+    https://github.com/pytest-dev/pytest/issues/9765
+    """
+    pytester.makepyprojecttoml(
+        """
+        [tool.pytest.ini_options]
+        addopts = "-p my_package.plugin.my_plugin"
+        """
+    )
+    pytester.makepyfile(
+        **{
+            "setup.py": (
+                """
+                from setuptools import setup
+
+                if __name__ == '__main__':
+                    setup(name='my_package', packages=['my_package', 'my_package.plugin'])
+                """
+            ),
+            "my_package/__init__.py": "",
+            "my_package/conftest.py": "",
+            "my_package/test_foo.py": "def test(): pass",
+            "my_package/plugin/__init__.py": "",
+            "my_package/plugin/my_plugin.py": (
+                """
+                import pytest
+
+                def pytest_configure(config):
+
+                    class SimplePlugin:
+                        @pytest.fixture(params=[1, 2, 3])
+                        def my_fixture(self, request):
+                            yield request.param
+
+                    config.pluginmanager.register(SimplePlugin())
+                """
+            ),
+        }
+    )
+
+    subprocess.run([sys.executable, "setup.py", "develop"], check=True)
+    try:
+        # We are using subprocess.run rather than pytester.run on purpose.
+        # pytester.run is adding the current directory to PYTHONPATH which avoids
+        # the bug. We also use pytest rather than python -m pytest for the same
+        # PYTHONPATH reason.
+        subprocess.run(
+            ["pytest", "my_package"], capture_output=True, check=True, text=True
+        )
+    except subprocess.CalledProcessError as exc:
+        raise AssertionError(
+            f"pytest command failed:\n{exc.stdout=!s}\n{exc.stderr=!s}"
+        ) from exc

testing/test_pluginmanager.py

@@ -98,6 +98,38 @@ class TestPytestPluginInteractions:
         config.pluginmanager.register(A())
         assert len(values) == 2
 
+    @pytest.mark.skipif(
+        not sys.platform.startswith("win"),
+        reason="requires a case-insensitive file system",
+    )
+    def test_conftestpath_case_sensitivity(self, pytester: Pytester) -> None:
+        """Unit test for issue #9765."""
+        config = pytester.parseconfig()
+        pytester.makepyfile(**{"tests/conftest.py": ""})
+
+        conftest = pytester.path.joinpath("tests/conftest.py")
+        conftest_upper_case = pytester.path.joinpath("TESTS/conftest.py")
+
+        mod = config.pluginmanager._importconftest(
+            conftest,
+            importmode="prepend",
+            rootpath=pytester.path,
+        )
+        plugin = config.pluginmanager.get_plugin(str(conftest))
+        assert plugin is mod
+
+        mod_uppercase = config.pluginmanager._importconftest(
+            conftest_upper_case,
+            importmode="prepend",
+            rootpath=pytester.path,
+        )
+        plugin_uppercase = config.pluginmanager.get_plugin(str(conftest_upper_case))
+        assert plugin_uppercase is mod_uppercase
+
+        # No str(conftestpath) normalization so conftest should be imported
+        # twice and modules should be different objects
+        assert mod is not mod_uppercase
+
     def test_hook_tracing(self, _config_for_test: Config) -> None:
         pytestpm = _config_for_test.pluginmanager  # fully initialized with plugins
         saveindent = []
@@ -368,7 +400,7 @@ class TestPytestPluginManager:
             pytester.makepyfile("pytest_plugins='xyz'"), root=pytester.path
         )
         with pytest.raises(ImportError):
-            pytestpm.consider_conftest(mod)
+            pytestpm.consider_conftest(mod, registration_name="unused")
 
 
 class TestPytestPluginManagerBootstrapming: