bpo-31588: Validate return value of __prepare__() methods (GH-3764)

orenmn · ncoghlan · commit 5837d0418f47 · 2017-09-28T00:04:37.000+10:00
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.
diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
@@ -864,6 +864,28 @@ def __prepare__(*args):
         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
diff --git a/Misc/NEWS.d/next/Core and Builtins/2017-09-26-13-03-16.bpo-31588.wT9Iy7.rst b/Misc/NEWS.d/next/Core and Builtins/2017-09-26-13-03-16.bpo-31588.wT9Iy7.rst
@@ -0,0 +1,2 @@
+Raise a `TypeError` with a helpful error message when class creation fails
+due to a metaclass with a bad ``__prepare__()`` method. Patch by Oren Milman.
diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c
@@ -157,6 +157,13 @@ builtin___build_class__(PyObject *self, PyObject **args, Py_ssize_t nargs,
         Py_DECREF(bases);
         return NULL;
     }
+    if (!PyMapping_Check(ns)) {
+        PyErr_Format(PyExc_TypeError,
+                     "%.200s.__prepare__() must return a mapping, not %.200s",
+                     isclass ? ((PyTypeObject *)meta)->tp_name : "<metaclass>",
+                     Py_TYPE(ns)->tp_name);
+        goto error;
+    }
     cell = PyEval_EvalCodeEx(PyFunction_GET_CODE(func), PyFunction_GET_GLOBALS(func), ns,
                              NULL, 0, NULL, 0, NULL, 0, NULL,
                              PyFunction_GET_CLOSURE(func));

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	+Raise a `TypeError` with a helpful error message when class creation fails
	`2`	+due to a metaclass with a bad ``__prepare__()`` method. Patch by Oren Milman.