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topological_sort(G, nbunch=None, reverse=False)[source]

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.

Parameters :

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.

Raises :


Topological sort is defined for directed graphs only. If the graph G is undirected, a NetworkXError is raised.


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 [R234] .


[R234](1, 2) Skiena, S. S. The Algorithm Design Manual (Springer-Verlag, 1998). http://www.amazon.com/exec/obidos/ASIN/0387948600/ref=ase_thealgorithmrepo/