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.