Fix module prefix matching by splitting dot-separated paths.

fiddle/_src/absl_flags/utils.py

@@ -117,7 +117,10 @@ def _import_dotted_name(
       # If e.name is a prefix of name_pieces, the path simply doesn't
       # exist at this split point. Otherwise, the module
       # exists but broke importing an unrelated dependency.
-      if (missing := e.name.split('.')) != name_pieces[: len(missing)]:
+      splitted_name_pieces = []
+      for pieces in name_pieces:
+        splitted_name_pieces += pieces.split('.')
+      if (missing := e.name.split('.')) != splitted_name_pieces[: len(missing)]:
         raise
       if i == 1:  # Final iteration through the loop.
         raise

fiddle/_src/absl_flags/utils_test.py

@@ -191,6 +191,30 @@ def test_nonexistent_module_raises_module_not_found(self):
           module=None,
       )
 
+  def test_dotted_module_prefix_matching(self):
+    """Test that dot-separated module paths are split correctly for matching."""
+    import types  # pylint: disable=g-import-not-at-top
+
+    parent_a = types.ModuleType('a')
+    sub_b = types.ModuleType('a.b')
+
+    class C:
+      d = 42
+
+    sub_b.c = C
+
+    sys.modules['a'] = parent_a
+    sys.modules['a.b'] = sub_b
+    self.addCleanup(lambda: sys.modules.pop('a', None))
+    self.addCleanup(lambda: sys.modules.pop('a.b', None))
+
+    result = utils._import_dotted_name(
+        'c.d',
+        mode=_IRRELEVANT_MODE,
+        module=sub_b,
+    )
+    self.assertEqual(result, 42)
+
 
 if __name__ == '__main__':
   absltest.main()