Consider the following graph structure (borrowed from this question):
G = networkx.DiGraph()
G.add_edges_from([('n', 'n1'), ('n', 'n2'), ('n', 'n3')])
G.add_edges_from([('n4', 'n41'), ('n1', 'n11'), ('n1', 'n12'), ('n1', 'n13')])
G.add_edges_from([('n2', 'n21'), ('n2', 'n22')])
G.add_edges_from([('n13', 'n131'), ('n22', 'n221')])
which yields:
n---->n1--->n11
| |--->n12
| |--->n13
| |--->n131
|--->n2
| |---->n21
| |---->n22
| |--->n221
|--->n3
I can perform a depth-first search for successors starting at node n
and get:
> dfs_successors(G, 'n')
{'n': ['n1', 'n2', 'n3'],
'n1': ['n12', 'n13', 'n11'],
'n13': ['n131'],
'n131': ['n221'],
'n2': ['n22', 'n21']}
However, when I do a depth-first search for predecessors at e.g. node n221
, nothing happens:
> dfs_predecessors(G, 'n221')
{}
I would expect the output to be:
{'n221': ['n22', 'n2', 'n']}
What is going wrong here, and how can I get my expected behaviour?