Raise ValueError on invalid parent

Signed-off-by: Filipe Laíns <lains@riseup.net>

Author: FFY00

Doc/library/importlib.rst

@@ -283,6 +283,8 @@ ABC hierarchy::
 
       Returns an iterable of possible specs.
 
+      Raises :exc:`ValueError` if *parent* is not a package module.
+
       .. warning::
          This method can potentially yield a very large number of objects, and
          it may carry out IO operations when computing these values.
@@ -335,6 +337,8 @@ ABC hierarchy::
 
       Returns an iterable of possible specs.
 
+      Raises :exc:`ValueError` if *parent* is not a package module.
+
       .. warning::
          This method can potentially yield a very large number of objects, and
          it may carry out IO operations when computing these values.

Lib/importlib/_bootstrap_external.py

@@ -1512,6 +1512,9 @@ def _find_children(self):
                     break
 
     def discover(self, parent=None):
+        if parent and parent.submodule_search_locations is None:
+            raise ValueError(f'{parent} is not a package module')
+
         module_prefix = f'{parent.name}.' if parent else ''
         for child_name in self._find_children():
             if spec := self.find_spec(module_prefix + child_name):