How to use the altgraph.GraphError function in altgraph

"""
    D = {}    # dictionary of final distances
    P = {}    # dictionary of predecessors
    Q = _priorityDictionary()    # estimated distances of non-final vertices
    Q[start] = 0

    for v in Q:
        D[v] = Q[v]
        if v == end: break

        for w in graph.out_nbrs(v):
            edge_id  = graph.edge_by_node(v,w)
            vwLength = D[v] + graph.edge_data(edge_id)
            if w in D:
                if vwLength < D[w]:
                    raise GraphError("Dijkstra: found better path to already-final vertex")
            elif w not in Q or vwLength < Q[w]:
                Q[w] = vwLength
                P[w] = v

    return (D,P)

if create_nodes:
            self.add_node(head_id)
            self.add_node(tail_id)

        # store edge information
        self.edges[edge] = (head_id, tail_id, edge_data)

        # update the corresponding incoming and outgoing lists in the nodes
        # index 0 -> incoming edges
        # index 1 -> outgoing edges

        try:
            self.nodes[tail_id][0].append(edge)
            self.nodes[head_id][1].append(edge)
        except KeyError:
            raise GraphError('Invalid nodes %s -> %s' % (head_id, tail_id))

        self.next_edge += 1

def out_edges(self, node):
        """
        Returns a list of the outgoing edges
        """
        try:
            return list(self.nodes[node][1])
        except KeyError:
            raise GraphError('Invalid node %s' % node)

        return None

def restore_edge(self, edge):
        """
        Restores a previously hidden edge back into the graph.
        """
        try:
            self.edges[edge] = head_id, tail_id, data = self.hidden_edges[edge]
            self.nodes[tail_id][0].append(edge)
            self.nodes[head_id][1].append(edge)
            del self.hidden_edges[edge]
        except KeyError:
            raise GraphError('Invalid edge %s' % edge)

def edge_style(self, head, tail, **kwargs):
        '''
        Modifies an edge style to the dot representation.
        '''
        if tail not in self.nodes:
            raise GraphError("invalid node %s" % (tail,))

        try:
            if tail not in self.edges[head]:
                self.edges[head][tail]= {}
            self.edges[head][tail] = kwargs
        except KeyError:
            raise GraphError("invalid edge  %s -> %s " % (head, tail) )

def hide_edge(self, edge):
        """
        Hides an edge from the graph. The edge may be unhidden at some later
        time.
        """
        try:
            head_id, tail_id, edge_data = self.hidden_edges[edge] = self.edges[edge]
            self.nodes[tail_id][0].remove(edge)
            self.nodes[head_id][1].remove(edge)
            del self.edges[edge]
        except KeyError:
            raise GraphError('Invalid edge %s' % edge)

def edge_by_id(self, edge):
        """
        Returns the edge that connects the head_id and tail_id nodes
        """
        try:
            head, tail, data =  self.edges[edge]
        except KeyError:
            head, tail = None, None
            raise GraphError('Invalid edge %s' % edge)

        return (head, tail)

def restore_edge(self, edge):
        """
        Restores a previously hidden edge back into the graph.
        """
        try:
            head_id, tail_id, data = self.hidden_edges[edge]
            self.nodes[tail_id][0].append(edge)
            self.nodes[head_id][1].append(edge)
            self.edges[edge] = head_id, tail_id, data
            del self.hidden_edges[edge]
        except KeyError:
            raise GraphError('Invalid edge %s' % edge)

def out_edges(self, node):
        """
        Returns a list of the outgoing edges
        """
        try:
            return list(self.nodes[node][1])
        except KeyError:
            raise GraphError('Invalid node %s' % node)

        return None

def hide_edge(self, edge):
        """
        Hides an edge from the graph. The edge may be unhidden at some later
        time.
        """
        try:
            head_id, tail_id, edge_data = self.hidden_edges[edge] = self.edges[edge]
            self.nodes[tail_id][0].remove(edge)
            self.nodes[head_id][1].remove(edge)
            del self.edges[edge]
        except KeyError:
            raise GraphError('Invalid edge %s' % edge)