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