Question

I’m having trouble trying to implement __eq__ for a Rect class I wrote as a C extension. I tried defining a method called __eq__, but Python seems to override it.

static PyObject *
Rect___eq__(Rect *self, PyObject *other)
{
    Rect *rect = (Rect *) other;
    if (self->x != rect->x || self->y != rect->y || 
            self->width != rect->width || self->height != rect->height) {
        Py_RETURN_FALSE;
    } else {
        Py_RETURN_TRUE;
    }
}

static PyMethodDef Rect_methods[] = {
    {"__eq__", (PyCFunction)Rect___eq__, METH_VARARGS,
     "Compare Rects" },
    {NULL}  /* Sentinel */
};

It seems no matter what I do, Python defaults to “is” behavior:

>>> a = Rect(1, 2, 3, 4)
>>> b = Rect(1, 2, 3, 4)
>>> a == b
False
>>> a == a
True

Answer 1

When working with new types defined in C, you need to define tp_richcompare. Below is an implementation of rich compare for a type that always compares larger than all other types (except itself):

static PyObject *
Largest_richcompare(PyObject *self, PyObject *other, int op)
{
    PyObject *result = NULL;

    if (UndefinedObject_Check(other)) {
        result = Py_NotImplemented;
    }
    else {
        switch (op) {
        case Py_LT:
            result = Py_False;
            break;
        case Py_LE:
            result = (LargestObject_Check(other)) ? Py_True : Py_False;
            break;
        case Py_EQ:
            result = (LargestObject_Check(other)) ? Py_True : Py_False;
            break;
        case Py_NE:
            result = (LargestObject_Check(other)) ? Py_False : Py_True;
            break;
        case Py_GT:
            result = (LargestObject_Check(other)) ? Py_False : Py_True;
            break;
        case Py_GE:
            result = Py_True;
            break;
        }
    }

    Py_XINCREF(result);
    return result;
}

If you are using Python 3.x, you add it to the type object like this:

(richcmpfunc)&Largest_richcompare,       /* tp_richcompare */

If you are using Python 2.x, there is an extra step involved. Rich comparisons were added during the lifetime of Python 2.x and for a few versions of Python, a C extension could optionally define tp_richcomare. To inform Python 2.x that your type implements rich comparisons, you need to modify tp_flags by or-ing in Py_TPFLAGS_HAVE_RICHCOMPARE.

Py_TPFLAGS_DEFAULT|Py_TPFLAGS_HAVE_RICH_COMPARE,        /* tp_flags */