topological_sort¶

topological_sort(G)[source]¶

Return a generator of nodes in topologically sorted 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
Returns:	topologically_sorted_nodes – An iterable of node names in topological sorted order.
Return type:	iterable
Raises:	`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 can also be raised if G is changed while the returned iterator is being processed. `RuntimeError` – If G is changed while the returned iterator is being processed.

Examples

To get the reverse order of the topological sort:

>>> DG = nx.DiGraph([(1, 2), (2, 3)])
>>> list(reversed(list(nx.topological_sort(DG))))
[3, 2, 1]

Notes

This algorithm is based on a description and proof in Introduction to algorithms - a creative approach [1] .