from copy import copy
__all__ = ['topsort']
[docs]def topsort(graph):
'''
Topologically sort a graph
The graph should be of the form ``{node: [list of nodes], ...}``.
'''
# make a copy so as not to destroy original
graph = dict((k, copy(v)) for k, v in graph.items())
# Use the standard algorithm for topological sorting:
# http://en.wikipedia.org/wiki/Topological_sorting
# List that will contain the sorted elements
sorted_items = []
# set of all nodes with no incoming edges:
no_incoming = {node for node, edges in graph.items() if len(edges)==0}
while len(no_incoming):
n = no_incoming.pop()
sorted_items.append(n)
# find nodes m with edges to n
outgoing = [m for m, edges in graph.items() if n in edges]
for m in outgoing:
graph[m].remove(n)
if len(graph[m])==0:
# no other dependencies
no_incoming.add(m)
if any([len(edges) > 0 for edges in graph.values()]):
raise ValueError('Cannot topologically sort cyclic graph.')
return sorted_items