python错误Errors与异常Exceptions

例1：

1 while True:
  2         try:
  3                 a = int(input("please input a number: "))
  4                 break
  5         except ValueError:
  6                 print("Not valid input, try again...")
           

例2：

1 try:
  2         f = open("myfile.txt", r)
  3 except OSError as err:
  4         print("OS error: {0}".format(err))
  5 except ValueError:
  6         print("Could not convert data to an integer")