code: def add_edge_attribute(self, edge, attr):
          
          self.edge_attr[edge] = self.edge_attributes(edge) + [attr]
          
          if (not self.DIRECTED and edge[0] != edge[1]):
              self.edge_attr[(edge[1],edge[0])] = self.edge_attributes((edge[1], edge[0])) + [attr]
docs: Add attribute to the given edge.
  
          @type  edge: edge
          @param edge: One edge.
  
          @type  attr: tuple
          @param attr: Node attribute specified as a tuple in the form (attribute, value).