Issue #23722: Initialize __class__ from type.__new__()

The __class__ cell used by zero-argument super() is now initialized
from type.__new__ rather than __build_class__, so class methods
relying on that will now work correctly when called from metaclass
methods during class creation.

Patch by Martin Teichmann.

2 files changed, 84 insertions, 2 deletions

diff --git a/Lib/importlib/_bootstrap_external.py b/Lib/importlib/_bootstrap_external.py
index 9a7e6ec..bfb89da 100644
--- a/Lib/importlib/_bootstrap_external.py
+++ b/Lib/importlib/_bootstrap_external.py
@@ -236,7 +236,8 @@ _code_type = type(_write_atomic.__code__)
 #     Python 3.6b1  3373 (add BUILD_STRING opcode #27078)
 #     Python 3.6b1  3375 (add SETUP_ANNOTATIONS and STORE_ANNOTATION opcodes
 #                         #27985)
-#     Python 3.6a1  3376 (simplify CALL_FUNCTIONs & BUILD_MAP_UNPACK_WITH_CALL)
+#     Python 3.6b1  3376 (simplify CALL_FUNCTIONs & BUILD_MAP_UNPACK_WITH_CALL)
+#     Python 3.6b1  3377 (set __class__ cell from type.__new__ #23722)
 #
 # MAGIC must change whenever the bytecode emitted by the compiler may no
 # longer be understood by older implementations of the eval loop (usually
@@ -245,7 +246,7 @@ _code_type = type(_write_atomic.__code__)
 # Whenever MAGIC_NUMBER is changed, the ranges in the magic_values array
 # in PC/launcher.c must also be updated.
 
-MAGIC_NUMBER = (3376).to_bytes(2, 'little') + b'\r\n'
+MAGIC_NUMBER = (3377).to_bytes(2, 'little') + b'\r\n'
 _RAW_MAGIC_NUMBER = int.from_bytes(MAGIC_NUMBER, 'little')  # For import.c
 
 _PYCACHE = '__pycache__'
diff --git a/Lib/test/test_super.py b/Lib/test/test_super.py
index b84863f..a7ceded 100644
--- a/Lib/test/test_super.py
+++ b/Lib/test/test_super.py
@@ -143,6 +143,87 @@ class TestSuper(unittest.TestCase):
                 return __class__
         self.assertIs(X.f(), X)
 
+    def test___class___new(self):
+        test_class = None
+
+        class Meta(type):
+            def __new__(cls, name, bases, namespace):
+                nonlocal test_class
+                self = super().__new__(cls, name, bases, namespace)
+                test_class = self.f()
+                return self
+
+        class A(metaclass=Meta):
+            @staticmethod
+            def f():
+                return __class__
+
+        self.assertIs(test_class, A)
+
+    def test___class___delayed(self):
+        test_namespace = None
+
+        class Meta(type):
+            def __new__(cls, name, bases, namespace):
+                nonlocal test_namespace
+                test_namespace = namespace
+                return None
+
+        class A(metaclass=Meta):
+            @staticmethod
+            def f():
+                return __class__
+
+        self.assertIs(A, None)
+
+        B = type("B", (), test_namespace)
+        self.assertIs(B.f(), B)
+
+    def test___class___mro(self):
+        test_class = None
+
+        class Meta(type):
+            def mro(self):
+                # self.f() doesn't work yet...
+                self.__dict__["f"]()
+                return super().mro()
+
+        class A(metaclass=Meta):
+            def f():
+                nonlocal test_class
+                test_class = __class__
+
+        self.assertIs(test_class, A)
+
+    def test___classcell___deleted(self):
+        class Meta(type):
+            def __new__(cls, name, bases, namespace):
+                del namespace['__classcell__']
+                return super().__new__(cls, name, bases, namespace)
+
+        class A(metaclass=Meta):
+            @staticmethod
+            def f():
+                __class__
+
+        with self.assertRaises(NameError):
+            A.f()
+
+    def test___classcell___reset(self):
+        class Meta(type):
+            def __new__(cls, name, bases, namespace):
+                namespace['__classcell__'] = 0
+                return super().__new__(cls, name, bases, namespace)
+
+        class A(metaclass=Meta):
+            @staticmethod
+            def f():
+                __class__
+
+        with self.assertRaises(NameError):
+            A.f()
+        self.assertEqual(A.__classcell__, 0)
+
     def test_obscure_super_errors(self):
         def f():
             super()