PyObject_IsInstance is not safe when used to check if the object is an instance of specified builtin type. Typical code: rc = PyObject_IsInstance(obj, &Someting_Type); if (rc < 0) return NULL; if (rc) { SometingObject *something = (SometingObject *)obj; something->some_field ... } The __class__ attribute can be modified and PyObject_IsInstance() can return true if the object has not layout compatible with specified structure. And even worse, __class__ can be dynamic property and PyObject_IsInstance() can execute arbitrary Python code, that can invalidate cached values of pointers and sizes in C code. More safe way would be to use PyObject_IsSubclass(). rc = PyObject_IsSubclass((Py_Object *)obj->ob_type, &Someting_Type); if (rc < 0) return NULL; if (rc) { SometingObject *something = (SometingObject *)obj; something->some_field ... } For example see issue24102 [1], issue24091 [2] and many other issues opened by pkt. [1] http://bugs.python.org/issue24102 [2] http://bugs.python.org/issue24091