don't understand MRO

[Uwe Mayer]

Hi,

I have a subclassed PyQt class:

class Node(object):
def move(self, x,y): pass

class CRhomb(QCanvasP olygon, Node): pass

$ python
v2.4.1

CRhomb.mro() [<class '__main__.CRhom b'>, <class 'qtcanvas.QCanv asPolygon'>, <class
'qtcanvas.QCanv asPolygonalItem '>, <class 'qtcanvas.QCanv asItem'>, <class
'qt.Qt'>, <type 'sip.wrapper'>, <class '__main__.Node' >, <type 'object'>]
a = CRhomb()
a.move(1,2)

This executes also Node.move(a, 1,2)
Why?

Because even QCanvasItem.mov e delegates the call to the derived object? But
qt.Qt does not have a move() method... how does it get passed on to Node?

Thanks in advance,
Ciao
Uwe

[Terry Reedy]

"Uwe Mayer" <me*****@hadiko .de> wrote in message
news:d9******** **@news2.rz.uni-karlsruhe.de...

I have a subclassed PyQt class:

class Node(object):
def move(self, x,y): pass

class CRhomb(QCanvasP olygon, Node): pass

$ python
v2.4.1

CRhomb.mro() [<class '__main__.CRhom b'>, <class 'qtcanvas.QCanv asPolygon'>, <class
'qtcanvas.QCanv asPolygonalItem '>, <class 'qtcanvas.QCanv asItem'>, <class
'qt.Qt'>, <type 'sip.wrapper'>, <class '__main__.Node' >, <type 'object'>]
For those who don't know, 'mro' stands for 'method resolution order'. The
method returns a list of classes (all except the first are base or super
classes of the first) in the order in which their dictionaries are searched
for method (or other attribute) names.
a = CRhomb()
a.move(1,2)
This executes also Node.move(a, 1,2)
Why?
In the absence of other information, I would presume that none of the other
classes have a move() method.
Because even QCanvasItem.mov e delegates the call to the derived object?
But
qt.Qt does not have a move() method... how does it get passed on to Node?

Are you sure that QCanvasItem has a move method? What results from

print qtcanvas.QCanva sItem.move # ?

If so, I would need to see its code to try to answer.

Terry J. Reedy

[Uwe Mayer]

Thursday 23 June 2005 19:22 pm Terry Reedy wrote:

[...]

In the absence of other information, I would presume that none of the
other classes have a move() method.
move() is implemented in the class qtcanvas.QCanva sItem
I checked the pyqt sources and it is linked via sip to the C++ object file.
In C++, QCanvasItem.mov e is delegated to QCanvasItem.mov eBy.

-- snip: C++ sources --
void QCanvasItem::mo ve( double x, double y ){
moveBy( x-myx, y-myy );
}

void QCanvasItem::mo veBy( double dx, double dy ){
if ( dx || dy ) {
removeFromChunk s();
myx += dx;
myy += dy;
addToChunks();
}
}
-- snip --
Are you sure that QCanvasItem has a move method? What results from

print qtcanvas.QCanva sItem.move # ? If so, I would need to see its code to try to answer.

import qtcanvas
qtcanvas.QCanva sItem.move

<built-in function move>

Here is a working portion which recreates the strange output:

-- snip --
from qtcanvas import *

class Node(object):
def move(self, x,y):
print "Node: move(%d,%d)"%(x ,y)

class Rhomb(QCanvasPo lygon, Node):
def __init__(self, parent):
QCanvasPolygon. __init__(self, parent)
Node.__init__(s elf)

print Rhomb.mro()
r = Rhomb(None)
r.move(1,2)
-- snip --

This prints:

[<class '__main__.Rhomb '>, <class 'qtcanvas.QCanv asPolygon'>, <class
'qtcanvas.QCanv asPolygonalItem '>, <class 'qtcanvas.QCanv asItem'>, <class
'qt.Qt'>, <type 'sip.wrapper'>, <class '__main__.Node' >, <type 'object'>]
Node: move(1,2)

Ciao
Uwe

[Martin Franklin]

Uwe Mayer wrote:

Thursday 23 June 2005 19:22 pm Terry Reedy wrote:

[...]

In the absence of other information, I would presume that none of the
other classes have a move() method.

move() is implemented in the class qtcanvas.QCanva sItem
I checked the pyqt sources and it is linked via sip to the C++ object file.
In C++, QCanvasItem.mov e is delegated to QCanvasItem.mov eBy.

-- snip: C++ sources --
void QCanvasItem::mo ve( double x, double y ){
moveBy( x-myx, y-myy );
}

void QCanvasItem::mo veBy( double dx, double dy ){
if ( dx || dy ) {
removeFromChunk s();
myx += dx;
myy += dy;
addToChunks();
}
}

I wonder if it is to do with the signature of these methods. they
accept two doubles and perhaps the python bindings do not automatically
convert from integers, therefore these methods are not called and the
rules of mro kick in (thus calling the python move method)


-- snip --

Are you sure that QCanvasItem has a move method? What results from

>print qtcanvas.QCanva sItem.move # ?

If so, I would need to see its code to try to answer.

import qtcanvas
qtcanvas.QC anvasItem.move

<built-in function move>

Here is a working portion which recreates the strange output:

-- snip --
from qtcanvas import *

class Node(object):
def move(self, x,y):
print "Node: move(%d,%d)"%(x ,y)

class Rhomb(QCanvasPo lygon, Node):
def __init__(self, parent):
QCanvasPolygon. __init__(self, parent)
Node.__init__(s elf)

print Rhomb.mro()
r = Rhomb(None)
r.move(1,2)
-- snip --

This prints:

[<class '__main__.Rhomb '>, <class 'qtcanvas.QCanv asPolygon'>, <class
'qtcanvas.QCanv asPolygonalItem '>, <class 'qtcanvas.QCanv asItem'>, <class
'qt.Qt'>, <type 'sip.wrapper'>, <class '__main__.Node' >, <type 'object'>]
Node: move(1,2)

Ciao
Uwe