pathlib: handle filenames starting with `.` in `module_name_from_path`

src/_pytest/pathlib.py

@@ -648,6 +648,11 @@ def module_name_from_path(path: Path, root: Path) -> str:
     if len(path_parts) >= 2 and path_parts[-1] == "__init__":
         path_parts = path_parts[:-1]
 
+    # Module names cannot contain ".", normalize them to "_". This prevents
+    # a directory having a "." in the name (".env.310" for example) causing extra intermediate modules.
+    # Also, important to replace "." at the start of paths, as those are considered relative imports.
+    path_parts = [x.replace(".", "_") for x in path_parts]
+
     return ".".join(path_parts)
 
 

testing/test_pathlib.py

@@ -584,6 +584,18 @@ class TestImportLibMode:
         result = module_name_from_path(tmp_path / "__init__.py", tmp_path)
         assert result == "__init__"
 
+        # Modules which start with "." are considered relative and will not be imported
+        # unless part of a package, so we replace it with a "_" when generating the fake module name.
+        result = module_name_from_path(tmp_path / ".env/tests/test_foo.py", tmp_path)
+        assert result == "_env.tests.test_foo"
+
+        # We want to avoid generating extra intermediate modules if some directory just happens
+        # to contain a "." in the name.
+        result = module_name_from_path(
+            tmp_path / ".env.310/tests/test_foo.py", tmp_path
+        )
+        assert result == "_env_310.tests.test_foo"
+
     def test_insert_missing_modules(
         self, monkeypatch: MonkeyPatch, tmp_path: Path
     ) -> None: