check local class namespace before reaching for cells (closes #17853)

3 files changed, 19 insertions, 1 deletions

diff --git a/Lib/importlib/_bootstrap.py b/Lib/importlib/_bootstrap.py
index 77c14bc..1a046c5 100644
--- a/Lib/importlib/_bootstrap.py
+++ b/Lib/importlib/_bootstrap.py
@@ -388,12 +388,14 @@ def _call_with_frames_removed(f, *args, **kwds):
 #     Python 3.3a4  3230 (revert changes to implicit __class__ closure)
 #     Python 3.4a1  3250 (evaluate positional default arguments before
 #                        keyword-only defaults)
+#     Python 3.4a1  3260 (add LOAD_CLASSDEREF; allow locals of class to override
+#                        free vars)
 #
 # MAGIC must change whenever the bytecode emitted by the compiler may no
 # longer be understood by older implementations of the eval loop (usually
 # due to the addition of new opcodes).
 
-_MAGIC_BYTES = (3250).to_bytes(2, 'little') + b'\r\n'
+_MAGIC_BYTES = (3260).to_bytes(2, 'little') + b'\r\n'
 _RAW_MAGIC_NUMBER = int.from_bytes(_MAGIC_BYTES, 'little')
 
 _PYCACHE = '__pycache__'
diff --git a/Lib/opcode.py b/Lib/opcode.py
index a639fe3..4657b86 100644
--- a/Lib/opcode.py
+++ b/Lib/opcode.py
@@ -179,6 +179,9 @@ def_op('LIST_APPEND', 145)
 def_op('SET_ADD', 146)
 def_op('MAP_ADD', 147)
 
+def_op('LOAD_CLASSDEREF', 148)
+hasfree.append(148)
+
 def_op('EXTENDED_ARG', 144)
 EXTENDED_ARG = 144
 
diff --git a/Lib/test/test_scope.py b/Lib/test/test_scope.py
index 129a18a..f4ed244 100644
--- a/Lib/test/test_scope.py
+++ b/Lib/test/test_scope.py
@@ -714,6 +714,19 @@ class ScopeTests(unittest.TestCase):
             global a
 
 
+    def testClassNamespaceOverridesClosure(self):
+        # See #17853.
+        x = 42
+        class X:
+            locals()["x"] = 43
+            y = x
+        self.assertEqual(X.y, 43)
+        class X:
+            locals()["x"] = 43
+            del x
+        self.assertFalse(hasattr(X, "x"))
+        self.assertEqual(x, 42)
+
 
 def test_main():
     run_unittest(ScopeTests)