def __init__(self, tree, focus=None):
        """
        :param tree: the tree to be displayed
        :type tree: Tree
        :param focus: position of node to be focussed initially.
            This has to be a valid position in the Tree.
            It defaults to the value of `Tree.root`.
        """
        self._tree = tree
        self._focus = focus or tree.root
        self.root = tree.root
def __init__ ( self , tree , focus = None ) : self . _tree = tree self . _focus = focus or tree . root self . root = tree . root
:param tree: the tree to be displayed
        :type tree: Tree
        :param focus: position of node to be focussed initially.
            This has to be a valid position in the Tree.
            It defaults to the value of `Tree.root`.