[issue17005] Add a topological sort algorithm

[Gareth Rees]

Gareth Rees <gdr at garethrees.org> added the comment:

Just to elaborate on what I mean by "bug magnet". (I'm sure Pablo understands this, but there may be other readers who would like to see it spelled out.)

Suppose that you have a directed graph represented as a mapping from a vertex to an iterable of its out-neighbours. Then the "obvious" way to get a total order on the vertices in the graph would be to generate the edges and pass them to topsort:

    def edges(graph):
        return ((v, w) for v, ww in graph.items() for w in ww)
    order = topsort(edges(graph))

This will appear to work fine if it is never tested with a graph that has isolated vertices (which would be an all too easy omission).

To handle isolated vertices you have to remember to write something like this:

    reversed_graph = {v: [] for v in graph}
    for v, ww in graph.items():
        for w in ww:
            reversed_graph[w].append(v)
    order = topsort(edges(graph)) + [
          v for v, ww in graph.items() if not ww and not reversed_graph[v]]

I think it likely that beginner programmers will forget to do this and be surprised later on when their total order is missing some of the vertices.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue17005>
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