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Author belopolsky belopolsky, christian.heimes, eric.smith, martin.panter, rhettinger, terry.reedy, tshepang 2019-01-17.00:43:56 -1.0 Yes <1547685836.53.0.0918508657671.issue17005@roundup.psfhosted.org>
Content
```Here is an implementation that I've used for years. It is somewhat shorter than the one in PR 11583:

class CycleError(LogicalError, ValueError):
"""dependencies cycle detected

"""

def tsort(pairs):
"""topological sort

Just like unix tsort(1)

>>> tsort([(1, 2), (7, 8), (8, 10), (7, 4), (2, 3), (4, 10)])
[1, 7, 2, 8, 4, 3, 10]
>>> try:
...     tsort([(1,2), (2,1)])
... except CycleError as e:
...     print(e)
([], Counter({1: 1, 2: 1}), {1: [2], 2: [1]})
"""
# inspired by http://mail.python.org/pipermail/python-list/1999-July/002831.html
successors = {}
predecessor_counts = collections.Counter()
for x, y in pairs:
successors.setdefault(x, []).append(y)
predecessor_counts.setdefault(x, 0)
predecessor_counts[y] += 1
ordered = [x for x in predecessor_counts
if predecessor_counts[x] == 0]
for x in ordered:
del predecessor_counts[x]
for y in successors.get(x, ()):
predecessor_counts[y] -= 1
if predecessor_counts[y] == 0:
ordered.append(y)
if predecessor_counts:
raise CycleError(ordered, predecessor_counts, successors)
return ordered```
History
Date User Action Args
2019-01-17 00:43:57belopolskysetrecipients: + belopolsky, rhettinger, terry.reedy, eric.smith, christian.heimes, tshepang, martin.panter
2019-01-17 00:43:56belopolskysetmessageid: <1547685836.53.0.0918508657671.issue17005@roundup.psfhosted.org>