don't understand MRO

Terry Reedy tjreedy at
Thu Jun 23 19:22:58 CEST 2005

"Uwe Mayer" <merkosh at> wrote in message 
news:d9eluf$ojc$1 at
> I have a subclassed PyQt class:
> class Node(object):
>        def move(self, x,y): pass
> class CRhomb(QCanvasPolygon, Node): pass
> $ python
> v2.4.1
>>>> CRhomb.mro()
> [<class '__main__.CRhomb'>, <class 'qtcanvas.QCanvasPolygon'>, <class
> 'qtcanvas.QCanvasPolygonalItem'>, <class 'qtcanvas.QCanvasItem'>, <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.move 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.QCanvasItem.move # ?
If so, I would need to see its code to try to answer.

Terry J. Reedy

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