How to use the pydot.pydot.Subgraph function in pydot

Subgraph instance will be returned.

        If one or more subgraphs exist with the same name, a list of
        Subgraph instances is returned.
        An empty list is returned otherwise.
        """

        match = list()

        if self.obj_dict['subgraphs'].has_key( name ):

            sgraphs_obj_dict = self.obj_dict['subgraphs'].get( name )

            for obj_dict_list in sgraphs_obj_dict:
                #match.extend( Subgraph( obj_dict = obj_d ) for obj_d in obj_dict_list )
                match.append( Subgraph( obj_dict = obj_dict_list ) )

        return match

graph.append( node.to_string()+'\n' )

            elif obj['type'] == 'edge':

                edge = Edge(obj_dict=obj)

                if self.obj_dict.get('simplify', False) and edge in edges_done:
                    continue

                graph.append( edge.to_string() + '\n' )
                edges_done.add(edge)

            else:

                sgraph = Subgraph(obj_dict=obj)

                graph.append( sgraph.to_string()+'\n' )

        graph.append( '}\n' )

        return ''.join(graph)

elif isinstance(src, (int, long)):
            edge = [ str(src) ]
        else:
            edge = [ src ]

        if	(self.get_parent_graph() and
            self.get_parent_graph().get_top_graph_type() and
            self.get_parent_graph().get_top_graph_type() == 'digraph' ):

            edge.append( '->' )

        else:
            edge.append( '--' )

        if isinstance(dst, frozendict):
            edge.append( Subgraph(obj_dict=dst).to_string() )
        elif isinstance(dst, (int, long)):
            edge.append( str(dst) )
        else:
            edge.append( dst )


        edge_attr = list()

        for attr, value in self.obj_dict['attributes'].iteritems():

            if value is not None:
                edge_attr.append( '%s=%s' % (attr, quote_if_necessary(value) ) )
            else:
                edge_attr.append( attr )

        edge_attr = ', '.join(edge_attr)

def get_subgraph_list(self):
        """Get the list of Subgraph instances.

        This method returns the list of Subgraph instances
        in the graph.
        """

        sgraph_objs = list()

        for sgraph, obj_dict_list in self.obj_dict['subgraphs'].iteritems():
                sgraph_objs.extend( [ Subgraph( obj_dict = obj_d ) for obj_d in obj_dict_list ] )

        return sgraph_objs

def add_subgraph(self, sgraph):
        """Adds an subgraph object to the graph.

        It takes a subgraph object as its only argument and returns
        None.
        """

        if not isinstance(sgraph, Subgraph) and not isinstance(sgraph, Cluster):
            raise TypeError('add_subgraph() received a non subgraph class object:' + str(sgraph))

        if self.obj_dict['subgraphs'].has_key(sgraph.get_name()):

            sgraph_list = self.obj_dict['subgraphs'][ sgraph.get_name() ]
            sgraph_list.append( sgraph.obj_dict )

        else:
            self.obj_dict['subgraphs'][ sgraph.get_name() ] = [ sgraph.obj_dict ]

        sgraph.set_sequence( self.get_next_sequence_number() )

        sgraph.set_parent_graph( self.get_parent_graph() )