# Source code for networkx.algorithms.traversal.depth_first_search

```
"""
==================
Depth-first search
==================
Basic algorithms for depth-first searching the nodes of a graph.
Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py
by D. Eppstein, July 2004.
"""
import networkx as nx
from collections import defaultdict
__author__ = """\n""".join(['Aric Hagberg <aric.hagberg@gmail.com>'])
__all__ = ['dfs_edges', 'dfs_tree',
'dfs_predecessors', 'dfs_successors',
'dfs_preorder_nodes','dfs_postorder_nodes',
'dfs_labeled_edges']
[docs]def dfs_edges(G, source=None):
"""Produce edges in a depth-first-search (DFS).
Parameters
----------
G : NetworkX graph
source : node, optional
Specify starting node for depth-first search and return edges in
the component reachable from source.
Returns
-------
edges: generator
A generator of edges in the depth-first-search.
Examples
--------
>>> G = nx.Graph()
>>> G.add_path([0,1,2])
>>> print(list(nx.dfs_edges(G,0)))
[(0, 1), (1, 2)]
Notes
-----
Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py
by D. Eppstein, July 2004.
If a source is not specified then a source is chosen arbitrarily and
repeatedly until all components in the graph are searched.
"""
if source is None:
# produce edges for all components
nodes = G
else:
# produce edges for components with source
nodes = [source]
visited=set()
for start in nodes:
if start in visited:
continue
visited.add(start)
stack = [(start,iter(G[start]))]
while stack:
parent,children = stack[-1]
try:
child = next(children)
if child not in visited:
yield parent,child
visited.add(child)
stack.append((child,iter(G[child])))
except StopIteration:
stack.pop()
[docs]def dfs_tree(G, source):
"""Return oriented tree constructed from a depth-first-search from source.
Parameters
----------
G : NetworkX graph
source : node, optional
Specify starting node for depth-first search.
Returns
-------
T : NetworkX DiGraph
An oriented tree
Examples
--------
>>> G = nx.Graph()
>>> G.add_path([0,1,2])
>>> T = nx.dfs_tree(G,0)
>>> print(T.edges())
[(0, 1), (1, 2)]
"""
T = nx.DiGraph()
if source is None:
T.add_nodes_from(G)
else:
T.add_node(source)
T.add_edges_from(dfs_edges(G,source))
return T
[docs]def dfs_predecessors(G, source=None):
"""Return dictionary of predecessors in depth-first-search from source.
Parameters
----------
G : NetworkX graph
source : node, optional
Specify starting node for depth-first search and return edges in
the component reachable from source.
Returns
-------
pred: dict
A dictionary with nodes as keys and predecessor nodes as values.
Examples
--------
>>> G = nx.Graph()
>>> G.add_path([0,1,2])
>>> print(nx.dfs_predecessors(G,0))
{1: 0, 2: 1}
Notes
-----
Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py
by D. Eppstein, July 2004.
If a source is not specified then a source is chosen arbitrarily and
repeatedly until all components in the graph are searched.
"""
return dict((t,s) for s,t in dfs_edges(G,source=source))
[docs]def dfs_successors(G, source=None):
"""Return dictionary of successors in depth-first-search from source.
Parameters
----------
G : NetworkX graph
source : node, optional
Specify starting node for depth-first search and return edges in
the component reachable from source.
Returns
-------
succ: dict
A dictionary with nodes as keys and list of successor nodes as values.
Examples
--------
>>> G = nx.Graph()
>>> G.add_path([0,1,2])
>>> print(nx.dfs_successors(G,0))
{0: [1], 1: [2]}
Notes
-----
Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py
by D. Eppstein, July 2004.
If a source is not specified then a source is chosen arbitrarily and
repeatedly until all components in the graph are searched.
"""
d = defaultdict(list)
for s,t in dfs_edges(G,source=source):
d[s].append(t)
return dict(d)
[docs]def dfs_postorder_nodes(G,source=None):
"""Produce nodes in a depth-first-search post-ordering starting
from source.
Parameters
----------
G : NetworkX graph
source : node, optional
Specify starting node for depth-first search and return edges in
the component reachable from source.
Returns
-------
nodes: generator
A generator of nodes in a depth-first-search post-ordering.
Examples
--------
>>> G = nx.Graph()
>>> G.add_path([0,1,2])
>>> print(list(nx.dfs_postorder_nodes(G,0)))
[2, 1, 0]
Notes
-----
Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py
by D. Eppstein, July 2004.
If a source is not specified then a source is chosen arbitrarily and
repeatedly until all components in the graph are searched.
"""
post=(v for u,v,d in nx.dfs_labeled_edges(G,source=source)
if d['dir']=='reverse')
# potential modification: chain source to end of post-ordering
# return chain(post,[source])
return post
[docs]def dfs_preorder_nodes(G, source=None):
"""Produce nodes in a depth-first-search pre-ordering starting
from source.
Parameters
----------
G : NetworkX graph
source : node, optional
Specify starting node for depth-first search and return edges in
the component reachable from source.
Returns
-------
nodes: generator
A generator of nodes in a depth-first-search pre-ordering.
Examples
--------
>>> G = nx.Graph()
>>> G.add_path([0,1,2])
>>> print(list(nx.dfs_preorder_nodes(G,0)))
[0, 1, 2]
Notes
-----
Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py
by D. Eppstein, July 2004.
If a source is not specified then a source is chosen arbitrarily and
repeatedly until all components in the graph are searched.
"""
pre=(v for u,v,d in nx.dfs_labeled_edges(G,source=source)
if d['dir']=='forward')
# potential modification: chain source to beginning of pre-ordering
# return chain([source],pre)
return pre
[docs]def dfs_labeled_edges(G, source=None):
"""Produce edges in a depth-first-search (DFS) labeled by type.
Parameters
----------
G : NetworkX graph
source : node, optional
Specify starting node for depth-first search and return edges in
the component reachable from source.
Returns
-------
edges: generator
A generator of edges in the depth-first-search labeled with 'forward',
'nontree', and 'reverse'.
Examples
--------
>>> G = nx.Graph()
>>> G.add_path([0,1,2])
>>> edges = (list(nx.dfs_labeled_edges(G,0)))
Notes
-----
Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py
by D. Eppstein, July 2004.
If a source is not specified then a source is chosen arbitrarily and
repeatedly until all components in the graph are searched.
"""
# Based on http://www.ics.uci.edu/~eppstein/PADS/DFS.py
# by D. Eppstein, July 2004.
if source is None:
# produce edges for all components
nodes = G
else:
# produce edges for components with source
nodes = [source]
visited = set()
for start in nodes:
if start in visited:
continue
yield start,start,{'dir':'forward'}
visited.add(start)
stack = [(start,iter(G[start]))]
while stack:
parent,children = stack[-1]
try:
child = next(children)
if child in visited:
yield parent,child,{'dir':'nontree'}
else:
yield parent,child,{'dir':'forward'}
visited.add(child)
stack.append((child,iter(G[child])))
except StopIteration:
stack.pop()
if stack:
yield stack[-1][0],parent,{'dir':'reverse'}
yield start,start,{'dir':'reverse'}
```