Python 判断 有向图 是否有环

import numpy
from numpy import *
def dfs( v ):
    vis[v] = -1
    flag = 0
    for i in range(n):
        # print (a[v][i],'---', vis[i] )
        if a[v][i] != 0  and vis[i] != -1:
            dfs(i)
            vis[i] = 1
        else:
            pass

        if a[v][i] != 0 and  vis[i] == -1:
            print ('Yes, there is A loop in this network\n')
            global swi
            swi = True
            exit()
            return
            # break
        else:
            pass
    print ('s = 0')
    return False


global swi
swi = False
'''===装载数据'''

edges = numpy.loadtxt('9_nodes_with_r_edge_8_to_3.txt')



# edges = [ int(i) for i in edges]
bian = int(shape(edges)[0]) - 1
print (bian,'edges in the network \n')
print (shape(edges),'\n')



n = int( edges[0][1] )


c = int( edges[0][0] )
# n, c = input().split()
# n = int(n)
# c = int(c)
a = [([0] * n) for i in range(n)]
vis = [0] * c