bpo-39336: Allow packages to not let their child modules be set on them

Lib/importlib/_bootstrap.py

@@ -978,7 +978,12 @@ def _find_and_load_unlocked(name, import_):
     if parent:
         # Set the module as an attribute on its parent.
         parent_module = sys.modules[parent]
-        setattr(parent_module, name.rpartition('.')[2], module)
+        child = name.rpartition('.')[2]
+        try:
+            setattr(parent_module, child, module)
+        except AttributeError:
+            msg = f"Cannot set an attribute on {parent!r} for child module {child!r}"
+            _warnings.warn(msg, ImportWarning)
     return module
 
 

Lib/test/test_import/__init__.py

@@ -26,6 +26,7 @@
     temp_dir, DirsOnSysPath)
 from test.support import script_helper
 from test.test_importlib.util import uncache
+from types import ModuleType
 
 
 skip_if_dont_write_bytecode = unittest.skipIf(
@@ -1339,6 +1340,19 @@ def test_circular_from_import(self):
             str(cm.exception),
         )
 
+    def test_unwritable_module(self):
+        self.addCleanup(unload, "test.test_import.data.unwritable")
+        self.addCleanup(unload, "test.test_import.data.unwritable.x")
+
+        import test.test_import.data.unwritable as unwritable
+        with self.assertWarns(ImportWarning):
+            from test.test_import.data.unwritable import x
+
+        self.assertNotEqual(type(unwritable), ModuleType)
+        self.assertEqual(type(x), ModuleType)
+        with self.assertRaises(AttributeError):
+            unwritable.x = 42
+
 
 if __name__ == '__main__':
     # Test needs to be a package, so we can do relative imports.

Lib/test/test_import/data/unwritable/__init__.py

@@ -0,0 +1,12 @@
+import sys
+
+class MyMod(object):
+    __slots__ = ['__builtins__', '__cached__', '__doc__',
+                 '__file__', '__loader__', '__name__',
+                 '__package__', '__path__', '__spec__']
+    def __init__(self):
+        for attr in self.__slots__:
+            setattr(self, attr, globals()[attr])
+
+
+sys.modules[__name__] = MyMod()

Misc/NEWS.d/next/Core and Builtins/2020-01-15-01-39-29.bpo-39336.nJ7W9I.rst

@@ -0,0 +1 @@
+Import loaders which publish immutable module objects can now publish immutable packages in addition to individual modules.