Catching Exceptions Using Try and Except

In previous example we’ve been using the raw_input and int method/function. The raw_input function is used for user input from the keyboard and int function is used to transform the user input which is always in str (string) into int type of value.  So in all these examples we had to type numbers if we had typed the string the function int wouldn’t work. The Python interpreter would give us a ValueError. Here’s an example:

>>>value = raw_input(‘Please enter a number: ‘)
Please enter a number: thing 
>>>x = int(value) 
Traceback (most recent call last):
  File "", line 1, in 
ValueError: invalid literal for int() with base 10: 'thing '

In order to avoid ValueError  we’ll implement the try and except commands.

>>>value = raw_input(‘Please enter a number: ‘) 
>>>try: 
… x = int(value)
… y = x + 5
… print y
…except:
… print “Please enter a number:”
...

Python starts by executing the sequence of statements in the try block. If all goes well, it skips the except block and proceeds. If an exception occurs in the try block in our case we’ve written string instead of integer, then Python jumps out of the try block and executes the sequence of statements in the except block.

In general, catching an exception gives you a chance to fix the problem, or try again, or at least end the program gracefully.