Warning

This documents an unmaintained version of NetworkX. Please upgrade to a maintained version and see the current NetworkX documentation.

# Source code for networkx.algorithms.traversal.edgebfs

"""
=============================
=============================

Algorithms for a depth-first traversal of edges in a graph.

"""
from collections import deque
import networkx as nx

FORWARD = 'forward'
REVERSE = 'reverse'

__all__ = ['edge_bfs']

[docs]def edge_bfs(G, source=None, orientation=None):
"""A directed, breadth-first-search of edges in G, beginning at source.

Yield the edges of G in a breadth-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 breadth-first-search.
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.

Examples
--------
>>> import networkx as nx
>>> nodes = [0, 1, 2, 3]
>>> edges = [(0, 1), (1, 0), (1, 0), (2, 0), (2, 1), (3, 1)]

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

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

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

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

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

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

Notes
-----
The goal of this function is to visit edges. It differs from the more
familiar breadth-first-search of nodes, as provided by
:func:networkx.algorithms.traversal.breadth_first_search.bfs_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.

--------
bfs_edges
bfs_tree
edge_dfs

"""
nodes = list(G.nbunch_iter(source))
if not nodes:
return

directed = G.is_directed()
kwds = {'data': False}
if G.is_multigraph() is True:
kwds['keys'] = True

# set up edge lookup
if orientation is None:
def edges_from(node):
return iter(G.edges(node, **kwds))
elif not directed or orientation == 'original':
def edges_from(node):
for e in G.edges(node, **kwds):
yield e + (FORWARD,)
elif orientation == 'reverse':
def edges_from(node):
for e in G.in_edges(node, **kwds):
yield e + (REVERSE,)
elif orientation == 'ignore':
def edges_from(node):
for e in G.edges(node, **kwds):
yield e + (FORWARD,)
for e in G.in_edges(node, **kwds):
yield e + (REVERSE,)
else:
raise nx.NetworkXError("invalid orientation argument.")

if directed:
neighbors = G.successors
def edge_id(edge):
# remove direction indicator
return edge[:-1] if orientation is not None else edge
else:
neighbors = G.neighbors
def edge_id(edge):
return (frozenset(edge[:2]),) +edge[2:]

check_reverse = directed and orientation in ('reverse', 'ignore')

# start BFS
visited_nodes = {n for n in nodes}
visited_edges = set()
queue = deque([(n, edges_from(n)) for n in nodes])
while queue:
parent, children_edges = queue.popleft()
for edge in children_edges:
if check_reverse and edge[-1] == REVERSE:
child = edge[0]
else:
child = edge[1]
if child not in visited_nodes: