什么是python Bellman-Ford算法

def bellman_ford( graph, source ):
    
    distance = {}
    parent = {}
    
    for node in graph:
        distance[node] = float( 'Inf' )
        parent[node] = None
    distance[source] = 0
 
    for i in range( len( graph ) - 1 ):
        for from_node in graph:
            for to_node in graph[from_node]:
                if distance[to_node] > graph[from_node][to_node] + distance[from_node]:
                    distance[to_node] = graph[from_node][to_node] + distance[from_node]
                    parent[to_node] = from_node
 
    for from_node in graph:
        for to_node in graph[from_node]:
            if distance[to_node] > distance[from_node] + graph[from_node][to_node]:
                return None, None
 
    return distance, parent
 
def test():
    graph = {
        'a': {'b': -1, 'c':  4},
        'b': {'c':  3, 'd':  2, 'e':  2},
        'c': {},
        'd': {'b':  1, 'c':  5},
        'e': {'d': -3}
    }
    distance, parent = bellman_ford( graph, 'a' )
    print distance
    print parent
 
if __name__ == '__main__':
    test()