topological_sort(G, nbunch=None, reverse=False)¶
Return a list of nodes in topological sort order.
A topological sort is a nonunique permutation of the nodes such that an edge from u to v implies that u appears before v in the topological sort order.
- G (NetworkX digraph) – A directed graph
- nbunch (container of nodes (optional)) – Explore graph in specified order given in nbunch
- reverse (bool, optional) – Return postorder instead of preorder if True. Reverse mode is a bit more efficient.
NetworkXError– Topological sort is defined for directed graphs only. If the graph G is undirected, a NetworkXError is raised.
NetworkXUnfeasible– If G is not a directed acyclic graph (DAG) no topological sort exists and a NetworkXUnfeasible exception is raised.
This algorithm is based on a description and proof in The Algorithm Design Manual  .
 Skiena, S. S. The Algorithm Design Manual (Springer-Verlag, 1998). http://www.amazon.com/exec/obidos/ASIN/0387948600/ref=ase_thealgorithmrepo/