graphs.py

import graphlib


def ts_sort(topological_sorter):
    topological_sorter.prepare()
    spaces = 0
    while topological_sorter:
        nodes = topological_sorter.get_ready()
        print(" "*spaces, nodes)
        for node in nodes:
            topological_sorter.done(node)
        spaces += 2

ts = graphlib.TopologicalSorter()
ts.add('s1', 's0')
ts.add('s1', 's2')
ts_sort(ts)