假设一段python程序有多重循环，我们都知道在一个循环当中，用break是退出当前的循环，然后继续下一次循环，但是如何才能跳出多重循环呢，实际就是结束所有的循环。
思路1：:可以定义一个异常类，在需要退出循环的位置，跑出这个异常，就可以了，因为程序在遇到异常的时候，如果没有捕获异常的话，就是直接退出程序的执行，利用这个原理，认为的触发一个异常，然后把这个异常捕获到，就可以实现退出多重循环，而且不结束程序，继续往下执行
#encoding=utf-8
class getOutOfLoop(Exception):
pass
try:
for i in range(5):
for j in range(5):
for k in range(5):
if i == j ==k ==3:
raise getOutOfLoop()#抛出一个异常，就会跳出所有循环
else:
print i,'---',j,'---',k
except getOutOfLoop:
pass
print "done"
结果：可以看到在循环到特定条件时，直接抛出异常，然后进行捕获，使程序跳出循环后并不结束，而是接着捕获异常的位置后面继续执行。
D:>python test.py
0 --- 0 --- 0
0 --- 0 --- 1
0 --- 0 --- 2
0 --- 0 --- 3
0 --- 0 --- 4
0 --- 1 --- 0
0 --- 1 --- 1
0 --- 1 --- 2
0 --- 1 --- 3
0 --- 1 --- 4
0 --- 2 --- 0
0 --- 2 --- 1
0 --- 2 --- 2
0 --- 2 --- 3
0 --- 2 --- 4
0 --- 3 --- 0
0 --- 3 --- 1
0 --- 3 --- 2
0 --- 3 --- 3
0 --- 3 --- 4
0 --- 4 --- 0
0 --- 4 --- 1
0 --- 4 --- 2
0 --- 4 --- 3
0 --- 4 --- 4
1 --- 0 --- 0
1 --- 0 --- 1
1 --- 0 --- 2
1 --- 0 --- 3
1 --- 0 --- 4
1 --- 1 --- 0
1 --- 1 --- 1
1 --- 1 --- 2
1 --- 1 --- 3
1 --- 1 --- 4
1 --- 2 --- 0
1 --- 2 --- 1
1 --- 2 --- 2
1 --- 2 --- 3
1 --- 2 --- 4
1 --- 3 --- 0
1 --- 3 --- 1
1 --- 3 --- 2
1 --- 3 --- 3
1 --- 3 --- 4
1 --- 4 --- 0
1 --- 4 --- 1
1 --- 4 --- 2
1 --- 4 --- 3
1 --- 4 --- 4
2 --- 0 --- 0
2 --- 0 --- 1
2 --- 0 --- 2
2 --- 0 --- 3
2 --- 0 --- 4
2 --- 1 --- 0
2 --- 1 --- 1
2 --- 1 --- 2
2 --- 1 --- 3
2 --- 1 --- 4
2 --- 2 --- 0
2 --- 2 --- 1
2 --- 2 --- 2
2 --- 2 --- 3
2 --- 2 --- 4
2 --- 3 --- 0
2 --- 3 --- 1
2 --- 3 --- 2
2 --- 3 --- 3
2 --- 3 --- 4
2 --- 4 --- 0
2 --- 4 --- 1
2 --- 4 --- 2
2 --- 4 --- 3
2 --- 4 --- 4
3 --- 0 --- 0
3 --- 0 --- 1
3 --- 0 --- 2
3 --- 0 --- 3
3 --- 0 --- 4
3 --- 1 --- 0
3 --- 1 --- 1
3 --- 1 --- 2
3 --- 1 --- 3
3 --- 1 --- 4
3 --- 2 --- 0
3 --- 2 --- 1
3 --- 2 --- 2
3 --- 2 --- 3
3 --- 2 --- 4
3 --- 3 --- 0
3 --- 3 --- 1
3 --- 3 --- 2
done
思路2：在多重循环中用return，直接进行返回，程序遇到return就会认为程序执行结束，进而退出多个循环，这种情况，执行return后，程序不会继续往下执行，而是直接结束程序
#encoding=utf-8
#函数中，用return会退出多重循环
def test():
for i in range(5):
for j in range(5):
for k in range(5):
if i ==j==k==3:
return
else:
print i,'---',j,'---',k
test()
结果：
D:>python test.py
0 --- 0 --- 0
0 --- 0 --- 1
0 --- 0 --- 2
0 --- 0 --- 3
0 --- 0 --- 4
0 --- 1 --- 0
0 --- 1 --- 1
0 --- 1 --- 2
0 --- 1 --- 3
0 --- 1 --- 4
0 --- 2 --- 0
0 --- 2 --- 1
0 --- 2 --- 2
0 --- 2 --- 3
0 --- 2 --- 4
0 --- 3 --- 0
0 --- 3 --- 1
0 --- 3 --- 2
0 --- 3 --- 3
0 --- 3 --- 4
0 --- 4 --- 0
0 --- 4 --- 1
0 --- 4 --- 2
0 --- 4 --- 3
0 --- 4 --- 4
1 --- 0 --- 0
1 --- 0 --- 1
1 --- 0 --- 2
1 --- 0 --- 3
1 --- 0 --- 4
1 --- 1 --- 0
1 --- 1 --- 1
1 --- 1 --- 2
1 --- 1 --- 3
1 --- 1 --- 4
1 --- 2 --- 0
1 --- 2 --- 1
1 --- 2 --- 2
1 --- 2 --- 3
1 --- 2 --- 4
1 --- 3 --- 0
1 --- 3 --- 1
1 --- 3 --- 2
1 --- 3 --- 3
1 --- 3 --- 4
1 --- 4 --- 0
1 --- 4 --- 1
1 --- 4 --- 2
1 --- 4 --- 3
1 --- 4 --- 4
2 --- 0 --- 0
2 --- 0 --- 1
2 --- 0 --- 2
2 --- 0 --- 3
2 --- 0 --- 4
2 --- 1 --- 0
2 --- 1 --- 1
2 --- 1 --- 2
2 --- 1 --- 3
2 --- 1 --- 4
2 --- 2 --- 0
2 --- 2 --- 1
2 --- 2 --- 2
2 --- 2 --- 3
2 --- 2 --- 4
2 --- 3 --- 0
2 --- 3 --- 1
2 --- 3 --- 2
2 --- 3 --- 3
2 --- 3 --- 4
2 --- 4 --- 0
2 --- 4 --- 1
2 --- 4 --- 2
2 --- 4 --- 3
2 --- 4 --- 4
3 --- 0 --- 0
3 --- 0 --- 1
3 --- 0 --- 2
3 --- 0 --- 3
3 --- 0 --- 4
3 --- 1 --- 0
3 --- 1 --- 1
3 --- 1 --- 2
3 --- 1 --- 3
3 --- 1 --- 4
3 --- 2 --- 0
3 --- 2 --- 1
3 --- 2 --- 2
3 --- 2 --- 3
3 --- 2 --- 4
3 --- 3 --- 0
3 --- 3 --- 1
3 --- 3 --- 2