Nested Cluster in pydot

I want to arrange the nodes in such a manner that there are 2 clusters A and B.

B has further 2 clusters C and D.

For the same i wrote the below program in which i am adding 2 clusters inside one and then adding cluster B as a subgraph in main graph.

However the nested boxes do not appear. Can someone tell me what am i doing wrong ?

import pydot

d开发者_如何学Goef draw( ListA , ListB , ListC , filename ):
    graph = pydot.Dot(graph_type='digraph',fontsize = 50  )
    List_nodesA = []
    List_nodesB = []
    List_nodesC = []
    cluster1 = pydot.Cluster( "A" , color = ".3 .5 .7"  )
    cluster2 = pydot.Cluster( "D" , color = ".6 .5 .2"  )
    cluster3 = pydot.Cluster( "C" , color = ".7 .5 .9" )
    cluster4 = pydot.Cluster( "B" )
    for item in ListA:
        List_nodesA.append(pydot.Node(str( item ) , shape = "circle", style="filled", fillcolor="0.5 0.4 0.9" ))
    for item in ListB:
        List_nodesB.append(pydot.Node(str( item ) , shape = "circle", style="filled", fillcolor="orange"))
    for item in ListC:
        List_nodesC.append(pydot.Node(str( item ) , shape = "circle", style="filled", fillcolor="green"))

    for node in List_nodesA:
        cluster1.add_node( node )

    graph.add_subgraph( cluster1 )
    for node in List_nodesB:
        cluster2.add_node( node )  # create cluster D
    cluster4.add_subgraph( cluster2 ) # add it to B

    for node in List_nodesC:
        cluster3.add_node( node ) # create cluster C
    cluster4.add_subgraph( cluster3 ) # add it to B

    graph.add_subgraph( cluster4 )
    for vertex1 in List_nodesA:
        for vertex2 in List_nodesB:
            graph.add_edge( pydot.Edge( vertex1 , vertex2 , len=1.5 ) )

    graph.write(filename,prog = 'neato',format = 'png')

draw( [1,2] , [3,4] , [5,6] , "graph.png" )

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Neato does not support clusters, as it uses a spring model where node placement is very dynamic. If you replace neato engine with dot engine, your clusters will appear correcly.