How to use the birdears.urwid.treetools.TreeWidgetError function in birdears

def load_parent(self):
        """Provide TreeNode with a parent for the current node.  This function
        is only required if the tree was instantiated from a child node
        (virtual function)"""
        raise TreeWidgetError("virtual function.  Implement in subclass")

def load_child_keys(self):
        """Provide ParentNode with an ordered list of child keys (virtual
        function)"""
        raise TreeWidgetError("virtual function.  Implement in subclass")

def load_child_node(self, key):
        """Load the child node for a given key (virtual function)"""
        raise TreeWidgetError("virtual function.  Implement in subclass")

def get_child_index(self, key):
        try:
            return self.get_child_keys().index(key)
        except ValueError:
            errorstring = ("Can't find key %s in ParentNode %s\n" +
                           "ParentNode items: %s")
            raise TreeWidgetError(errorstring % (key, self.get_key(),
                                  str(self.get_child_keys())))