1, b=int(3.3)
   print b     这个能够正确输出3

2,b=raw_input()
  a=int(b)
  print a     假如输入3.3，也能够正确输入3.3

3,a=int(raw_input())
  print a     假如输入3.3却提示错误
Traceback (most recent call last):
  File "C:/Python26/1.py", line 1, in <module>
    a=int(raw_input())
ValueError: invalid literal for int() with base 10: '3.3'

请问大侠们，第三种情况为何错误呢？