Fix modutils._is_subpath to handle base with trailing os.sep

ChangeLog

@@ -42,6 +42,10 @@ What's New in astroid 4.0.4?
 ============================
 Release date: TBA
 
+* Fix ability to detect .py modules inside PATH directories on Windows
+  described by a UNC path with a trailing backslash (`\`)
+    - Example: modutils.modpath_from_file(filename=r"\\Mac\Code\tests\test_resources.py", path=["\\mac\code\"]) == ['tests', 'test_resources']
+
 
 
 What's New in astroid 4.0.3?

astroid/modutils.py

@@ -237,7 +237,11 @@ def _is_subpath(path: str, base: str) -> bool:
     base = os.path.normcase(os.path.normpath(base))
     if not path.startswith(base):
         return False
-    return (len(path) == len(base)) or (path[len(base)] == os.path.sep)
+    return (
+        (len(path) == len(base))
+        or (path[len(base)] == os.path.sep)
+        or (base.endswith(os.path.sep) and path[len(base) - 1] == os.path.sep)
+    )
 
 
 def _get_relative_base_path(filename: str, path_to_check: str) -> list[str] | None:

tests/test_modutils.py

@@ -151,6 +151,31 @@ def test_get_module_part_only_dot(self) -> None:
         self.assertEqual(modutils.get_module_part(".", modutils.__file__), ".")
 
 
+class IsSubpathTest(unittest.TestCase):
+    """Tests for modutils._is_subpath,
+    which is used internally by modutils.modpath_from_file."""
+
+    @unittest.skipUnless(sys.platform == "win32", "Windows-specific path test")
+    def test_is_subpath_with_trailing_separator_unc_path(self) -> None:
+        self.assertTrue(
+            modutils._is_subpath(
+                "\\\\Mac\\Code\\tests\\test_resources.py",
+                # UNC path with trailing separator
+                "\\\\mac\\code\\",
+            )
+        )
+
+    @unittest.skipUnless(sys.platform == "win32", "Windows-specific path test")
+    def test_is_subpath_without_trailing_separator_unc_path(self) -> None:
+        self.assertTrue(
+            modutils._is_subpath(
+                "\\\\Mac\\Code\\tests\\test_resources.py",
+                # UNC path without trailing separator
+                "\\\\mac\\code",
+            )
+        )
+
+
 class ModPathFromFileTest(unittest.TestCase):
     """Given an absolute file path return the python module's path as a list."""