python异常

[root@localhost python]# cat try.py
#!/usr/bin/python

while True:
      try:
           x=int(raw_input("Try num: "));
           break;
      except  ValueError:
           print "Valid number...";

while True:
           x=int(raw_input("No Try num: "));

[root@localhost python]# ./try.py
［注］try
Try num: d
Valid number...
Try num: d
Valid number...
Try num: 3

[注]无try
No Try num: 3
No Try num: 3
No Try num: d
Traceback (most recent call last):
  File "./try.py", line 12, in ?
    x=int(raw_input("No Try num: "));
ValueError: invalid literal for int(): d