diff -r 2b7b203e3909 Lib/ctypes/test/test_simplesubclasses.py
--- a/Lib/ctypes/test/test_simplesubclasses.py	Wed Jan 11 20:18:03 2017 +0200
+++ b/Lib/ctypes/test/test_simplesubclasses.py	Sat Jan 14 02:12:36 2017 +0000
@@ -7,6 +7,10 @@
             return NotImplementedError
         return self.value == other.value
 
+    # See issue 29270.
+    def __repr__(self):
+        return super().__repr__()
+
 class Test(unittest.TestCase):
 
     def test_compare(self):
diff -r 2b7b203e3909 Modules/_ctypes/_ctypes.c
--- a/Modules/_ctypes/_ctypes.c	Wed Jan 11 20:18:03 2017 +0200
+++ b/Modules/_ctypes/_ctypes.c	Sat Jan 14 02:12:36 2017 +0000
@@ -1771,6 +1771,13 @@
     PyObject *swapped_args;
     static PyObject *suffix;
     Py_ssize_t i;
+    PyObject *dict = PyTuple_GET_ITEM(args, 2);
+    _Py_IDENTIFIER(__classcell__);
+
+    /* Prevent PyType_Type.tp_new from overwriting the primary class that
+     * was already set in __classcell__. See issue 29270. */
+    if (_PyDict_DelItemId(dict, &PyId___classcell__))
+        PyErr_Clear(); 
 
     swapped_args = PyTuple_New(PyTuple_GET_SIZE(args));
     if (!swapped_args)