class GraphicScene(QGraphicScene):

    myNodes = dict()

    def addItem(node):
        self.myNodes[id(node.shape)] = node
        self.items.append(node.shape)


    def removeItem(shape):
        self.myNodes.remove(id(shape))
        self.items.remove(shape)

class NodeItem(object):

    shape = QPolygonF()