for i in range (1,4):
   print(" ")
   print("外层第%s遍循环: "%i)
   for j in range(1, 4):
      print("内层第%s遍循环: "%j,end=" ")
   print(" ")   

• 1
• 2
• 3
• 4
• 5
• 6

结果

其实就是如下的遍历：
(1,1)(1,2)(1,3)
(2,1)(2,2)(2,3)
(3,1)(3,2)(3,3)