pallindrome script

I wrote the below python script to find if the string is pallindrome or not.

while True:
    try:
        input_string = str(input("Enter a string: "))
        if input_string.isdigit():
            pass
        else:
            break
    except Exception, e:
        print e
a = []
b = []
for i in input_string:
    a.append(i)
a.reverse()

for i in input_string:
    b.append(i)
if a == b:
    print input_string, "is a pallindrome"
else:
    print input_string, "is not a pallindrome"

Output:
[madhu@localhost tmp]$ python pallindrome.py
 Enter a string: "elle"
 elle is a pallindrome
 [madhu@localhost tmp]$ python pallindrome.py
 Enter a string: "malayalam"
 malayalam is a pallindrome
 [madhu@localhost tmp]$ python pallindrome.py
 Enter a string: "pallindrome"
 pallindrome is not a pallindrome