Try to import module before creating dummy modules with 'importmode=importlib'

The dummy modules we introduce in `insert_missing_modules` (due to #7856 and #7859)
would cause problems if the dummy modules actually end up replacing modules
which could be imported normally because they are available in `PYTHONPATH`.

Now we attempt to first import the module via normal mechanisms, and only
introduce the dummy modules if the intermediary modules don't actually exist.

Close #9645

changelog/9645.bugfix.rst

@@ -0,0 +1 @@
+Fixed regression where ``--import-mode=importlib`` used together with :envvar:`PYTHONPATH` or :confval:`pythonpath` would cause import errors in test suites.

src/_pytest/pathlib.py

@@ -603,6 +603,15 @@ def insert_missing_modules(modules: Dict[str, ModuleType], module_name: str) ->
     module_parts = module_name.split(".")
     while module_name:
         if module_name not in modules:
+            try:
+                # If sys.meta_path is empty, calling import_module will issue
+                # a warning and raise ModuleNotFoundError. To avoid the
+                # warning, we check sys.meta_path explicitly and raise the error
+                # ourselves to fall back to creating a dummy module.
+                if not sys.meta_path:
+                    raise ModuleNotFoundError
+                importlib.import_module(module_name)
+            except ModuleNotFoundError:
                 module = ModuleType(
                     module_name,
                     doc="Empty module created by pytest's importmode=importlib.",

testing/test_collection.py

@@ -1507,6 +1507,35 @@ class TestImportModeImportlib:
             ]
         )
 
+    def test_using_python_path(self, pytester: Pytester) -> None:
+        """
+        Dummy modules created by insert_missing_modules should not get in
+        the way of modules that could be imported via python path (#9645).
+        """
+        pytester.makeini(
+            """
+            [pytest]
+            pythonpath = .
+            addopts = --import-mode importlib
+            """
+        )
+        pytester.makepyfile(
+            **{
+                "tests/__init__.py": "",
+                "tests/conftest.py": "",
+                "tests/subpath/__init__.py": "",
+                "tests/subpath/helper.py": "",
+                "tests/subpath/test_something.py": """
+                import tests.subpath.helper
+
+                def test_something():
+                    assert True
+                """,
+            }
+        )
+        result = pytester.runpytest()
+        result.stdout.fnmatch_lines("*1 passed in*")
+
 
 def test_does_not_crash_on_error_from_decorated_function(pytester: Pytester) -> None:
     """Regression test for an issue around bad exception formatting due to

testing/test_pathlib.py

@@ -562,15 +562,20 @@ class TestImportLibMode:
         result = module_name_from_path(Path("/home/foo/test_foo.py"), Path("/bar"))
         assert result == "home.foo.test_foo"
 
-    def test_insert_missing_modules(self) -> None:
-        modules = {"src.tests.foo": ModuleType("src.tests.foo")}
-        insert_missing_modules(modules, "src.tests.foo")
-        assert sorted(modules) == ["src", "src.tests", "src.tests.foo"]
+    def test_insert_missing_modules(
+        self, monkeypatch: MonkeyPatch, tmp_path: Path
+    ) -> None:
+        monkeypatch.chdir(tmp_path)
+        # Use 'xxx' and 'xxy' as parent names as they are unlikely to exist and
+        # don't end up being imported.
+        modules = {"xxx.tests.foo": ModuleType("xxx.tests.foo")}
+        insert_missing_modules(modules, "xxx.tests.foo")
+        assert sorted(modules) == ["xxx", "xxx.tests", "xxx.tests.foo"]
 
         mod = ModuleType("mod", doc="My Module")
-        modules = {"src": mod}
-        insert_missing_modules(modules, "src")
-        assert modules == {"src": mod}
+        modules = {"xxy": mod}
+        insert_missing_modules(modules, "xxy")
+        assert modules == {"xxy": mod}
 
         modules = {}
         insert_missing_modules(modules, "")