I'm encountering some strange behavior and hope someone on this list
can provide some insight. We're using pygtk 0.6.8 with python 2.1,
and gtk-canvas 0.1. We're using the gtk-canvas instead of
gnome-canvas so that our users don't have to install all of gnome. A
disadvantage of that is that we have to create our own python wrappers
for the canvas functions. Everything was fine until I tried to
extract mouse click coordinates.
Here's what happens. There's a C++ class, OOFCanvas, that wraps the
gtk-canvas. It has a set_callback() function which looks like this:
void OOFCanvas::set_callback(PyObject *callback) {
pycallback = callback; // pycallback is OOFCanvas instance data
Py_XINCREF(callback);
gtk_signal_connect(GTK_OBJECT(root), // root is the gtk_canvas_root
"event",
GTK_SIGNAL_FUNC(OOFCanvas::mouse_event),
this);
}
set_callback() is SWIGged, and I know that its argument is being
handled correctly, since I can call the callback under other
circumstances. (The SWIG typemap uses PyCallable_Check to verify that
the argument is sane.)
When the canvas gets an event, it calls the static function
OOFCanvas::mouse_event, which just calls a non-static function:
gint OOFCanvas::mouse_event(GtkCanvasItem *item, GdkEvent *event, gpointer data)
{
OOFCanvas *oofcanvas = (OOFCanvas*)(data);
oofcanvas->mouse_eventCB(item, event);
return TRUE;
}
That non-static function invokes the Python callback:
void OOFCanvas::mouse_eventCB(GtkCanvasItem *item, GdkEvent *event) {
PyObject *args = 0;
switch(event->type) {
case GDK_MOTION_NOTIFY:
args = Py_BuildValue("(sdd)", "motion_notify",
event->motion.x, event->motion.y);
break;
// [ several other event types omitted ]
}
if(args) {
PyObject *result = PyEval_CallObject(pycallback, args); // dumps core!
Py_XDECREF(args);
Py_XDECREF(result);
}
}
The call to PyEval_CallObject() dumps core. I know that the argument
is being constructed correctly, since I can print it with
PyString_AsString(PyObject_Repr(args)). I know that the callback is
set correctly, since I can examine it in the same way. I can even
call the callback function successfully (but not usefully) if I call
it from set_callback(). It just doesn't work when I call it from
within a GtkCanvasItem signal callback!
Is there something special that I have to do to when calling Python
from C within a C gtk signal callback? I can call the same Python
function successfully, with the same C++ code, as long as I don't call
it from within the signal handler.
Any advice would be much appreciated.
-- Steve
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