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# edge_dfs

edge_dfs(G, source=None, orientation=None)

  A directed, depth-first-search of edges in `G`, beginning at `source`.

  Yield the edges of G in a depth-first-search order continuing until all edges are generated.

  Parameters:
    G graph

      A directed/undirected graph/multigraph.

    source node, list of nodes

      The node from which the traversal begins. If None, then a source is chosen arbitrarily and repeatedly until all edges from each node in the graph are searched.

    orientation None | ‘original’ | ‘reverse’ | ‘ignore’ (default: None)

      For directed graphs and directed multigraphs, edge traversals need not respect the original orientation of the edges. When set to ‘reverse’ every edge is traversed in the reverse direction. When set to ‘ignore’, every edge is treated as undirected. When set to ‘original’, every edge is treated as directed. In all three cases, the yielded edge tuples add a last entry to indicate the direction in which that edge was traversed. If orientation is None, the yielded edge has no direction indicated. The direction is respected, but not reported.

  Yields:
    edge directed edge

      A directed edge indicating the path taken by the depth-first traversal. For graphs, `edge` is of the form `(u, v)` where `u` and `v` are the tail and head of the edge as determined by the traversal. For multigraphs, `edge` is of the form `(u, v, key)`, where `key` is the key of the edge. When the graph is directed, then `u` and `v` are always in the order of the actual directed edge. If orientation is not None then the edge tuple is extended to include the direction of traversal (‘forward’ or ‘reverse’) on that edge.

  See also

  `dfs_edges()`

  Notes

  The goal of this function is to visit edges. It differs from the more familiar depth-first traversal of nodes, as provided by `dfs_edges()`, in that it does not stop once every node has been visited. In a directed graph with edges [(0, 1), (1, 2), (2, 1)], the edge (2, 1) would not be visited if not for the functionality provided by this function.

  Examples

      >>> nodes = [0, 1, 2, 3]
      >>> edges = [(0, 1), (1, 0), (1, 0), (2, 1), (3, 1)]

      >>> list(nx.edge_dfs(nx.Graph(edges), nodes))
      [(0, 1), (1, 2), (1, 3)]

      >>> list(nx.edge_dfs(nx.DiGraph(edges), nodes))
      [(0, 1), (1, 0), (2, 1), (3, 1)]

      >>> list(nx.edge_dfs(nx.MultiGraph(edges), nodes))
      [(0, 1, 0), (1, 0, 1), (0, 1, 2), (1, 2, 0), (1, 3, 0)]

      >>> list(nx.edge_dfs(nx.MultiDiGraph(edges), nodes))
      [(0, 1, 0), (1, 0, 0), (1, 0, 1), (2, 1, 0), (3, 1, 0)]

      >>> list(nx.edge_dfs(nx.DiGraph(edges), nodes, orientation="ignore"))
      [(0, 1, 'forward'), (1, 0, 'forward'), (2, 1, 'reverse'), (3, 1, 'reverse')]

      >>> list(nx.edge_dfs(nx.MultiDiGraph(edges), nodes, orientation="ignore"))
      [(0, 1, 0, 'forward'), (1, 0, 0, 'forward'), (1, 0, 1, 'reverse'), (2, 1, 0, 'reverse'), (3, 1, 0, 'reverse')]