dfs_labeled_edges(G, source=None, depth_limit=None, *, sort_neighbors=None)[source]#

Iterate over edges in a depth-first-search (DFS) labeled by type.

GNetworkX graph
sourcenode, optional

Specify starting node for depth-first search and return edges in the component reachable from source.

depth_limitint, optional (default=len(G))

Specify the maximum search depth.

sort_neighborsfunction (default=None)

A function that takes an iterator over nodes as the input, and returns an iterable of the same nodes with a custom ordering. For example, sorted will sort the nodes in increasing order.

edges: generator

A generator of triples of the form (u, v, d), where (u, v) is the edge being explored in the depth-first search and d is one of the strings ‘forward’, ‘nontree’, ‘reverse’, or ‘reverse-depth_limit’. A ‘forward’ edge is one in which u has been visited but v has not. A ‘nontree’ edge is one in which both u and v have been visited but the edge is not in the DFS tree. A ‘reverse’ edge is one in which both u and v have been visited and the edge is in the DFS tree. When the depth_limit is reached via a ‘forward’ edge, a ‘reverse’ edge is immediately generated rather than the subtree being explored. To indicate this flavor of ‘reverse’ edge, the string yielded is ‘reverse-depth_limit’.


If a source is not specified then a source is chosen arbitrarily and repeatedly until all components in the graph are searched.

The implementation of this function is adapted from David Eppstein’s depth-first search function in PADS, with modifications to allow depth limits based on the Wikipedia article “Depth-limited search”.


The labels reveal the complete transcript of the depth-first search algorithm in more detail than, for example, dfs_edges():

>>> from pprint import pprint
>>> G = nx.DiGraph([(0, 1), (1, 2), (2, 1)])
>>> pprint(list(nx.dfs_labeled_edges(G, source=0)))
[(0, 0, 'forward'),
 (0, 1, 'forward'),
 (1, 2, 'forward'),
 (2, 1, 'nontree'),
 (1, 2, 'reverse'),
 (0, 1, 'reverse'),
 (0, 0, 'reverse')]