Validate that __length_hint__ returns a usable result.

Lib/test/test_iterlen.py

@@ -208,6 +208,11 @@ class BadLengthHint(object):
     def __length_hint__(self):
         raise RuntimeError('hello')
 
+class NoneLengthHint(object):
+    def __iter__(self): return iter(range(10))
+    def __length_hint__(self):
+        return None
+
 class TestLengthHintExceptions(unittest.TestCase):
 
     def test_issue1242657(self):
@@ -219,6 +224,11 @@ class TestLengthHintExceptions(unittest.TestCase):
         self.assertRaises(RuntimeError, b.extend, BadLen())
         self.assertRaises(RuntimeError, b.extend, BadLengthHint())
 
+    def test_invalid_hint(self):
+        # Make sure an invalid result doesn't muck-up the works
+        self.assertEqual(list(NoneLengthHint()), list(range(10)))
+
+
 def test_main():
     unittests = [
         TestRepeat,

Objects/abstract.c

@@ -105,7 +105,7 @@ _PyObject_LengthHint(PyObject *o, Py_ssize_t defaultvalue)
 		PyErr_Clear();
 		return defaultvalue;
 	}
-	rv = PyLong_AsSsize_t(ro);
+	rv = PyLong_Check(ro) ? PyLong_AsSsize_t(ro) : defaultvalue;
 	Py_DECREF(ro);
 	return rv;
 }