Harden get_module_part() against "." (#2202) (#2203)

(cherry picked from commit bd78ab0)

Co-authored-by: Jacob Walls <jacobtylerwalls@gmail.com>

ChangeLog

@@ -19,6 +19,15 @@ Release date: TBA
 
 
 
+What's New in astroid 2.15.6?
+=============================
+Release date: 2023-05-14
+
+* Harden ``get_module_part()`` against ``"."``.
+
+  Closes pylint-dev/pylint#8749
+
+
 What's New in astroid 2.15.5?
 =============================
 Release date: 2023-05-14

astroid/modutils.py

@@ -433,7 +433,8 @@ def get_module_part(dotted_name: str, context_file: str | None = None) -> str:
         ), "explicit relative import, but no context_file?"
         path = []  # prevent resolving the import non-relatively
         starti = 1
-    while parts[starti] == "":  # for all further dots: change context
+    # for all further dots: change context
+    while starti < len(parts) and parts[starti] == "":
         starti += 1
         assert (
             context_file is not None

tests/test_modutils.py

@@ -147,6 +147,9 @@ def test_get_module_part_exception(self) -> None:
             ImportError, modutils.get_module_part, "unknown.module", modutils.__file__
         )
 
+    def test_get_module_part_only_dot(self) -> None:
+        self.assertEqual(modutils.get_module_part(".", modutils.__file__), ".")
+
 
 class ModPathFromFileTest(unittest.TestCase):
     """Given an absolute file path return the python module's path as a list."""