[3.6] bpo-31588: Validate return value of __prepare__() methods (GH-3790)

Class execution requires that __prepare__() methods return
a proper execution namespace. Check for that immediately
after calling __prepare__(), rather than passing it through
to the code execution machinery and potentially triggering
SystemError (in debug builds) or a cryptic TypeError
(in release builds).

Patch by Oren Milman.
(cherry picked from commit 5837d0418f47933b2e3c139bdee8a79c248a943c)

1 files changed, 22 insertions, 0 deletions

diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
index 382ca03..d79429f 100644
--- a/Lib/test/test_types.py
+++ b/Lib/test/test_types.py
@@ -846,6 +846,28 @@ class ClassCreationTests(unittest.TestCase):
         self.assertIs(ns, expected_ns)
         self.assertEqual(len(kwds), 0)
 
+    def test_bad___prepare__(self):
+        # __prepare__() must return a mapping.
+        class BadMeta(type):
+            @classmethod
+            def __prepare__(*args):
+                return None
+        with self.assertRaisesRegex(TypeError,
+                                    r'^BadMeta\.__prepare__\(\) must '
+                                    r'return a mapping, not NoneType$'):
+            class Foo(metaclass=BadMeta):
+                pass
+        # Also test the case in which the metaclass is not a type.
+        class BadMeta:
+            @classmethod
+            def __prepare__(*args):
+                return None
+        with self.assertRaisesRegex(TypeError,
+                                    r'^<metaclass>\.__prepare__\(\) must '
+                                    r'return a mapping, not NoneType$'):
+            class Bar(metaclass=BadMeta()):
+                pass
+
     def test_metaclass_derivation(self):
         # issue1294232: correct metaclass calculation
         new_calls = []  # to check the order of __new__ calls