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bpo-31588: Validate return value of __prepare__() methods (GH-3764)
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.
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@ -864,6 +864,28 @@ def __prepare__(*args):
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self.assertIs(ns, expected_ns)
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self.assertEqual(len(kwds), 0)
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def test_bad___prepare__(self):
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# __prepare__() must return a mapping.
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class BadMeta(type):
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@classmethod
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def __prepare__(*args):
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return None
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with self.assertRaisesRegex(TypeError,
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r'^BadMeta\.__prepare__\(\) must '
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r'return a mapping, not NoneType$'):
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class Foo(metaclass=BadMeta):
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pass
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# Also test the case in which the metaclass is not a type.
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class BadMeta:
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@classmethod
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def __prepare__(*args):
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return None
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with self.assertRaisesRegex(TypeError,
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r'^<metaclass>\.__prepare__\(\) must '
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r'return a mapping, not NoneType$'):
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class Bar(metaclass=BadMeta()):
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pass
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def test_metaclass_derivation(self):
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# issue1294232: correct metaclass calculation
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new_calls = [] # to check the order of __new__ calls
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@ -0,0 +1,2 @@
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Raise a `TypeError` with a helpful error message when class creation fails
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due to a metaclass with a bad ``__prepare__()`` method. Patch by Oren Milman.
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@ -157,6 +157,13 @@ builtin___build_class__(PyObject *self, PyObject **args, Py_ssize_t nargs,
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Py_DECREF(bases);
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return NULL;
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}
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if (!PyMapping_Check(ns)) {
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PyErr_Format(PyExc_TypeError,
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"%.200s.__prepare__() must return a mapping, not %.200s",
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isclass ? ((PyTypeObject *)meta)->tp_name : "<metaclass>",
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Py_TYPE(ns)->tp_name);
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goto error;
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}
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cell = PyEval_EvalCodeEx(PyFunction_GET_CODE(func), PyFunction_GET_GLOBALS(func), ns,
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NULL, 0, NULL, 0, NULL, 0, NULL,
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PyFunction_GET_CLOSURE(func));
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