bpo-39336: Allow packages to not let their child modules be set on them (python#18006)

* bpo-39336: Allow setattr to fail on modules which aren't assignable

When attaching a child module to a package if the object in sys.modules raises an AttributeError (e.g. because it is immutable) it causes the whole import to fail.  This now allows immutable packages to exist and an ImportWarning is reported and the AttributeError exception is ignored.

Author: DinoV

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()

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

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