Pre-mark pytest_plugins for assertion rewrite

changelog/2353.bugfix.rst

@@ -0,0 +1 @@
+``pytest_plugins`` entries imported as a side effect of another plugin now also get assertion rewriting.

src/_pytest/config/__init__.py

@@ -909,6 +909,18 @@ def _import_plugin_specs(
         self, spec: types.ModuleType | str | Sequence[str] | None
     ) -> None:
         plugins = _get_plugin_specs_as_list(spec)
+        # Pre-mark plugins so side-effect imports are rewritten too (#2353).
+        # Skip blocked, registered or already imported ones. Their own import
+        # handles the warning.
+        self.rewrite_hook.mark_rewrite(
+            *(
+                _resolve_plugin_import_spec(plugin)
+                for plugin in plugins
+                if _resolve_plugin_import_spec(plugin) not in sys.modules
+                and not self.is_blocked(plugin)
+                and self.get_plugin(plugin) is None
+            )
+        )
         for import_spec in plugins:
             self.import_plugin(import_spec, consider_entry_points=True)
 
@@ -928,7 +940,7 @@ def import_plugin(self, modname: str, consider_entry_points: bool = False) -> No
         if self.is_blocked(modname) or self.get_plugin(modname) is not None:
             return
 
-        importspec = "_pytest." + modname if modname in builtin_plugins else modname
+        importspec = _resolve_plugin_import_spec(modname)
         self.rewrite_hook.mark_rewrite(importspec)
 
         if consider_entry_points:
@@ -1006,6 +1018,11 @@ def _get_plugin_specs_as_list(
     )
 
 
+def _resolve_plugin_import_spec(modname: str) -> str:
+    """Resolve a plugin name to its import name (builtins live under ``_pytest``)."""
+    return "_pytest." + modname if modname in builtin_plugins else modname
+
+
 def _iter_rewritable_modules(package_files: Iterable[str]) -> Iterator[str]:
     """Given an iterable of file names in a source distribution, return the "names" that should
     be marked for assertion rewrite.

testing/test_assertrewrite.py

@@ -1350,6 +1350,28 @@ def test_rewrite_warning_using_pytest_plugins(self, pytester: Pytester) -> None:
         result.stdout.fnmatch_lines(["*= 1 passed in *=*"])
         result.stdout.no_fnmatch_line("*pytest-warning summary*")
 
+    def test_plugin_imported_as_side_effect_is_rewritten(
+        self, pytester: Pytester
+    ) -> None:
+        """A plugin imported as a side effect of another plugin must still
+        get assertion rewriting (#2353)."""
+        pytester.makepyfile(
+            **{
+                "conftest.py": "pytest_plugins = ['plugin_2353_a', 'plugin_2353_b']",
+                "plugin_2353_a.py": "import plugin_2353_b",
+                "plugin_2353_b.py": "def check():\n    x = 1\n    assert x == 2\n",
+                "test_2353_side_effect.py": (
+                    "import plugin_2353_b\n\ndef test_rewritten():\n"
+                    "    plugin_2353_b.check()\n"
+                ),
+            }
+        )
+        pytester.chdir()
+        result = pytester.runpytest_subprocess()
+        result.assert_outcomes(failed=1)
+        result.stdout.fnmatch_lines(["E *assert 1 == 2*"])
+        result.stdout.no_fnmatch_line("*already imported*")
+
     def test_rewrite_warning_using_pytest_plugins_env_var(
         self, pytester: Pytester, monkeypatch
     ) -> None: