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

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.

Answer 2

This is an example how to integrate PyQt4 and boost::python

first of all we must define wrap/unwrap function to deal with bare pointers

long int unwrap(QObject* ptr) {
    return reinterpret_cast<long int>(ptr);
}

template <typename T>
T* wrap(long int ptr) {
    return reinterpret_cast<T*>(ptr);
}

after that we must register all classes we want integrate to

class_<QObject, QObject*, boost::noncopyable>("QObject", no_init)
    .def("unwrap", unwrap)
    .def("wrap", make_function( wrap<QObject>, return_value_policy<return_by_value>() ))
    .staticmethod("wrap");

class_<QWidget, bases<QObject>, QWidget*, boost::noncopyable>("QWidget")
    .def("wrap", make_function( wrap<QWidget>, return_value_policy<return_by_value>() ))
    .staticmethod("wrap");

class_<QFrame, bases<QWidget>, QFrame*, boost::noncopyable>("QFrame")
    .def("wrap", make_function( wrap<QFrame>, return_value_policy<return_by_value>() ))
    .staticmethod("wrap");

class_<QLabel, bases<QFrame>, QLabel*, boost::noncopyable>("QLabel")
    .def("wrap", make_function( wrap<QLabel>, return_value_policy<return_by_value>() ))
    .staticmethod("wrap");

and for example we have class that works with.. QLabel:

class worker: public QObject {
...
void add_label(QLabel*);
};

we must expose this class to python too:

class_<worker, bases<QObject>, worker*, boost::noncopyable>("worker")
        .def("add_label", &worker::add_label);

now we a ready to interaction, on C++-size do something like this

worker* w = new worker;
main_namespace["worker"] = boost::ref(w);

python:

from PyQt4.Qt import *
import sip
import mylib as MyLib

#...

#If you are using QApplication on C++-size you don't need to create another one

lb = QLabel("label from PyQt4!")

lb_ptr = sip.unwrapinstance(f)

my_lb = MyLib.QLabel.wrap(lb_ptr)

worker.add_label(my_lb)

In other case if you wan't send you own Q-object to PyQt4 :

QLabel* lb = new QLabel("C++ label");
main_namespace["lb"] = boost::ref(lb);

python:

from PyQt4.Qt import *
import sip
import mylib as MyLib

#...

my_lb_ptr = lb.unwrap()

qt_lb = sip.wrapinstance(my_lb_ptr, QLabel)

And this is my real little helper:

from PyQt4.Qt import *
import sip

def toQt(object, type):
    ptr = object.unwrap()
    return sip.wrapinstance(ptr, type)

def fromQt(object, type):
    ptr = sip.unwrapinstance(object)
    return type.wrap(ptr)

Answer 3

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!