Index: Lib/test/test_iter.py
===================================================================
--- Lib/test/test_iter.py	(revision 66030)
+++ Lib/test/test_iter.py	(working copy)
@@ -120,6 +120,13 @@
     def test_seq_class_iter(self):
         self.check_iterator(iter(SequenceClass(10)), range(10))
 
+    # Test a new_style class with __iter__ but no next() method
+    def test_new_style_iter_class(self):
+        class IterClass(object):
+            def __iter__(self):
+                return self
+        self.assertRaises(TypeError, iter, IterClass())
+
     # Test two-argument iter() with callable instance
     def test_iter_callable(self):
         class C:
@@ -877,7 +884,22 @@
         self.assertEqual(list(b), zip(range(5), range(5)))
         self.assertEqual(list(b), [])
 
+    def test_3720(self):
+        # Avoid a crash, when an iterator deletes its next() method.
+        class BadIterator(object):
+            def __iter__(self):
+                return self
+            def next(self):
+                del BadIterator.next
+                return 1
 
+        try:
+            for i in BadIterator() :
+                pass
+        except TypeError:
+            pass
+
+
 def test_main():
     run_unittest(TestCase)
 
Index: Include/object.h
===================================================================
--- Include/object.h	(revision 66030)
+++ Include/object.h	(working copy)
@@ -472,6 +472,7 @@
 PyAPI_FUNC(int) PyObject_HasAttr(PyObject *, PyObject *);
 PyAPI_FUNC(PyObject **) _PyObject_GetDictPtr(PyObject *);
 PyAPI_FUNC(PyObject *) PyObject_SelfIter(PyObject *);
+PyAPI_FUNC(PyObject *) PyObject_NextNotImplemented(PyObject *);
 PyAPI_FUNC(PyObject *) PyObject_GenericGetAttr(PyObject *, PyObject *);
 PyAPI_FUNC(int) PyObject_GenericSetAttr(PyObject *,
 					      PyObject *, PyObject *);
Index: Include/abstract.h
===================================================================
--- Include/abstract.h	(revision 66030)
+++ Include/abstract.h	(working copy)
@@ -636,7 +636,8 @@
 
 #define PyIter_Check(obj) \
     (PyType_HasFeature((obj)->ob_type, Py_TPFLAGS_HAVE_ITER) && \
-     (obj)->ob_type->tp_iternext != NULL)
+     (obj)->ob_type->tp_iternext != NULL && \
+     (obj)->ob_type->tp_iternext != &PyObject_NextNotImplemented)
 
      PyAPI_FUNC(PyObject *) PyIter_Next(PyObject *);
      /* Takes an iterator object and calls its tp_iternext slot,
Index: Objects/object.c
===================================================================
--- Objects/object.c	(revision 66030)
+++ Objects/object.c	(working copy)
@@ -1286,6 +1286,15 @@
 	return obj;
 }
 
+PyObject *
+PyObject_NextNotImplemented(PyObject *self)
+{
+	PyErr_Format(PyExc_TypeError,
+		     "'%.200s' object is not iterable",
+		     Py_TYPE(self)->tp_name);
+	return NULL;
+}
+
 /* Generic GetAttr functions - put these in your tp_[gs]etattro slot */
 
 PyObject *
Index: Objects/typeobject.c
===================================================================
--- Objects/typeobject.c	(revision 66030)
+++ Objects/typeobject.c	(working copy)
@@ -6012,8 +6012,12 @@
 	}
 	do {
 		descr = _PyType_Lookup(type, p->name_strobj);
-		if (descr == NULL)
+		if (descr == NULL) {
+			if (ptr == (void**)&type->tp_iternext) {
+				specific = PyObject_NextNotImplemented;
+			}
 			continue;
+		}
 		if (Py_TYPE(descr) == &PyWrapperDescr_Type) {
 			void **tptr = resolve_slotdups(type, p->name_strobj);
 			if (tptr == NULL || tptr == ptr)
Index: Objects/abstract.c
===================================================================
--- Objects/abstract.c	(revision 66030)
+++ Objects/abstract.c	(working copy)
@@ -3056,7 +3056,6 @@
 PyIter_Next(PyObject *iter)
 {
 	PyObject *result;
-	assert(PyIter_Check(iter));
 	result = (*iter->ob_type->tp_iternext)(iter);
 	if (result == NULL &&
 	    PyErr_Occurred() &&
Index: Modules/itertoolsmodule.c
===================================================================
--- Modules/itertoolsmodule.c	(revision 66030)
+++ Modules/itertoolsmodule.c	(working copy)
@@ -886,7 +886,6 @@
 	long ok;
 	PyObject *(*iternext)(PyObject *);
 
-	assert(PyIter_Check(it));
 	iternext = *Py_TYPE(it)->tp_iternext;
 	for (;;) {
 		item = iternext(it);
@@ -1031,7 +1030,6 @@
 	if (lz->stop == 1)
 		return NULL;
 
-	assert(PyIter_Check(it));
 	item = (*Py_TYPE(it)->tp_iternext)(it);
 	if (item == NULL)
 		return NULL;
@@ -1218,7 +1216,6 @@
 	Py_ssize_t oldnext;
 	PyObject *(*iternext)(PyObject *);
 
-	assert(PyIter_Check(it));
 	iternext = *Py_TYPE(it)->tp_iternext;
 	while (lz->cnt < lz->next) {
 		item = iternext(it);
@@ -1229,7 +1226,6 @@
 	}
 	if (lz->stop != -1 && lz->cnt >= lz->stop)
 		return NULL;
-	assert(PyIter_Check(it));
 	item = iternext(it);
 	if (item == NULL)
 		return NULL;
@@ -1361,7 +1357,6 @@
 	PyObject *result;
 	PyObject *it = lz->it;
 
-	assert(PyIter_Check(it));
 	args = (*Py_TYPE(it)->tp_iternext)(it);
 	if (args == NULL)
 		return NULL;
@@ -2585,7 +2580,6 @@
 	long ok;
 	PyObject *(*iternext)(PyObject *);
 
-	assert(PyIter_Check(it));
 	iternext = *Py_TYPE(it)->tp_iternext;
 	for (;;) {
 		item = iternext(it);
@@ -2729,7 +2723,6 @@
 	long ok;
 	PyObject *(*iternext)(PyObject *);
 
-	assert(PyIter_Check(it));
 	iternext = *Py_TYPE(it)->tp_iternext;
 	for (;;) {
 		item = iternext(it);
@@ -3067,7 +3060,6 @@
 		Py_INCREF(result);
 		for (i=0 ; i < tuplesize ; i++) {
 			it = PyTuple_GET_ITEM(lz->ittuple, i);
-			assert(PyIter_Check(it));
 			item = (*Py_TYPE(it)->tp_iternext)(it);
 			if (item == NULL) {
 				Py_DECREF(result);
@@ -3083,7 +3075,6 @@
 			return NULL;
 		for (i=0 ; i < tuplesize ; i++) {
 			it = PyTuple_GET_ITEM(lz->ittuple, i);
-			assert(PyIter_Check(it));
 			item = (*Py_TYPE(it)->tp_iternext)(it);
 			if (item == NULL) {
 				Py_DECREF(result);
@@ -3419,7 +3410,6 @@
                                 Py_INCREF(lz->fillvalue);
                                 item = lz->fillvalue;
                         } else {
-                                assert(PyIter_Check(it));
                                 item = (*Py_TYPE(it)->tp_iternext)(it);
                                 if (item == NULL) {
                                         lz->numactive -= 1;      
@@ -3448,7 +3438,6 @@
                                 Py_INCREF(lz->fillvalue);
                                 item = lz->fillvalue;
                         } else {
-                                assert(PyIter_Check(it));
                                 item = (*Py_TYPE(it)->tp_iternext)(it);
                                 if (item == NULL) {
                                         lz->numactive -= 1;