How to use "try" ... "except"

Any program will be given over to error checking.
Python provides an exception handling capability.
There are two parts : "error checking" "catching exceptions".
Now let's try a simple example:

>>> try :
...     input_str = int(input ("string "))
... except StandardError :
...     print " This is not a number"
... 
string 12
>>> try :
...     input_str = int(input ("string "))
... except StandardError :
...     print " This is not a number"
... 
string aaa
 This is not a number

The code :

input_str = int(input ("string "))

The code readed input will be convert in integer .
If types a value that's not an integer, the exception is caught.
In this case will print " This is not a number ".
This is very important because it generates more errors while running a program.
So, simply try to perform you action, and define what's to be done.
On except block if the action you want can't be completed.