human
I wrote the below code

    color = dict()
    order = []
    def dfs_walk(node):
        color[node] = 'grey'
        for succ in graph.successors(node):
            if (color.get(succ) == 'grey'):
                print 'CYCLE:    {0}-->{1}'.format(node, succ)
            if (succ not in color):
                dfs_walk(succ)
        order.append(node)
        color[node] = 'black'
    dfs_walk(root)
    return order

What's a good function header?
 ... Give me a Python code to print 'CYCLE:    {0}-->{1}'?
assistant
#Here is Python code to print 'CYCLE:    {0}-->{1}'.

def postorder_3color(graph, root):