networkx py

import networkx as nx

def getLeaves():
    return [v for v, d in g.out_degree if d == 0]

g = nx.DiGraph()
g.add_node("root")
g.add_node("mod-a")
g.add_node("mod-b")
g.add_node("lib-a")
g.add_node("lib-b")
g.add_node("lib-c")

g.add_node("base")
g.add_edge("root", "mod-a")
g.add_edge("root", "mod-b")
g.add_edge("mod-a", "lib-a")
g.add_edge("mod-a", "lib-b")
g.add_edge("mod-b", "lib-a")
g.add_edge("mod-b", "lib-c")
g.add_edge("lib-a", "base")

g.add_edge("lib-b", "base")

leaves = getLeaves()

while len(leaves) != 0:
    print(leaves)
    print(g.nodes)
    for leaf in leaves:
        print("remove " + leaf)
        g.remove_node(leaf)
    leaves = getLeaves()
    print()

print(leaves)