Hi. Example test.py: class A():     def __add__(self, var):         print("I'm in A class")         return 5 a = A() a+1 1+a Execution: python test.py I'm in A class Traceback (most recent call last):   File "../../test.py", line 7, in <module>     1+a TypeError: unsupported operand type(s) for +: 'int' and 'instance' So adding integer to class works fine, but adding class to integer fails. I could not understand why it happens. In objects/abstact.c we have the following function: static PyObject * binary_op1(PyObject *v, PyObject *w, const int op_slot) {     PyObject *x;     binaryfunc slotv = NULL;     binaryfunc slotw = NULL;     if (v->ob_type->tp_as_number != NULL)         slotv = NB_BINOP(v->ob_type->tp_as_number, op_slot);     if (w->ob_type != v->ob_type &&         w->ob_type->tp_as_number != NULL) {         slotw = NB_BINOP(w->ob_type->tp_as_number, op_slot);         if (slotw == slotv)             slotw = NULL;     }     if (slotv) {         if (slotw && PyType_IsSubtype(w->ob_type, v->ob_type)) {             x = slotw(v, w);             if (x != Py_NotImplemented)                 return x;             Py_DECREF(x); /* can't do it */             slotw = NULL;         }         x = slotv(v, w);         if (x != Py_NotImplemented)             return x;         Py_DECREF(x); /* can't do it */     }     if (slotw) {         x = slotw(v, w);         if (x != Py_NotImplemented)             return x;         Py_DECREF(x); /* can't do it */     }     Py_RETURN_NOTIMPLEMENTED; } When we adding class to integer we have both slotv and slotw. x = slotv(v, w); -> returns Py_NotImplemented. But in this case we should execute x = slotw(v, w); and function should be completed in the same way as when we adding integer to class. Can someone advise please where I mistake. -- thanks, Igor Vasilyev