'Anchors the item at its local itemPos to the item\'s parent at parentPos.
Both positions are expressed in values relative to the size of the item or parent;
a value of 0 indicates left or top edge, while 1 indicates right or bottom edge.
Optionally, offset may be specified to introduce an absolute offset.
Example: anchor a box such that its upper-right corner is fixed 10px left
and 10px down from its parent\'s upper-right corner::
box.anchor(itemPos=(1,0), parentPos=(1,0), offset=(-10,10))'

def anchor(self, itemPos, parentPos, offset=(0, 0)):

    parent = self.parentItem()
    if (parent is None):
        raise Exception('Cannot    anchor;    parent    is    not    set.')
    if (self.__parent is not parent):
        if (self.__parent is not None):
            self.__parent.geometryChanged.disconnect(self.__geometryChanged)
        self.__parent = parent
        parent.geometryChanged.connect(self.__geometryChanged)
    self.__itemAnchor = itemPos
    self.__parentAnchor = parentPos
    self.__offset = offset
    self.__geometryChanged()
