Python method resolution mystery

Question

I can't figure out why this program is failing.

#!/usr/bin/env python
from __future__ import division, print_function
from future_builtins import *
import types
import libui as ui
from PyQt4 import QtCore
import sip

p = ui.QPoint()
q = QtCore.QPoint()

def _q_getattr(self, attr):
    print("get %s" % attr)
    value = getattr(sip.wrapinstance(self.myself(), QtCore.QPoint), attr)
    print("get2 %s returned %s" % (attr, value))
    return value

p.__getattr__ = types.MethodType(_q_getattr, p)

print(p.__getattr__('x')())  # Works!  Prints "0"
print(p.x())  # AttributeError: 'QPoint' object has no attribute 'x'

I used Boost.Python to create libui, which exposes the class QPoint. I aso included PyQt4, which has a sip-exposed QPoint. I'm trying to accomplish a mapping between the two types.

I checked that p is a new-style class, so why isn't __getattr__ being called for p.x()?

Answer 1

This is somewhat similar to the issue someone else has encountered just yesterday. In short, it seems like special methods (like __getattr__, __str__, __repr__, __call__ and so on) aren't overridable in new-style class instance, i.e. you can only define them in its type.

And here's an adaptation of my solution for that problem which should hopefully work for yours:

def _q_getattr(self, attr):
    print("get %s" % attr)
    return getattr(self, 'x')

def override(p, methods):
    oldType = type(p)
    newType = type(oldType.__name__ + "_Override", (oldType,), methods)
    p.__class__ = newType

override(p, { '__getattr__': _q_getattr})
print(p.__getattr__('x')())  # Works!  Prints "0"
print(p.x())                 # Should work!

Answer 2

I suggest that you not attempt to expose QPoint in boost python. You should be able to register converters to/from python with boost that will use the SIP api functions to convert QPoint from/to python as the sip objects.

I've done it, but not recently enough to give more details.