cyclic_detection.py

def has_directed_cycle(graph):
    for node in graph.iterkeys():
        frontline = [node]
        was = set([node])
        while frontline:
            curr = frontline.pop()
            if curr in was:
                continue
            was.add(curr)
            for neighbor in graph[curr]:
                if neighbor == node:
                    return True
                else:
                    frontline.add(neighbor)
    return False


if __name__ == "__main__":
    node_count, edge_count = map(int, raw_input().strip().split())
    graph = [set() for _ in xrange(node_count)]
    for _ in xrange(edge_count):
        fr, to = map(int, raw_input().strip().split())
        graph[fr].add(to)
    print has_directed_cycle(graph)