An error that occurs during the execution of a computer program is known as exception.
Exceptions are known to non-programmers as instances that do not conform to a general rule.
The code, which harbours or blink the risk of an exception, is fixed in a try block

Let's take a simple example. Lets ask the user to enter an integer number. If we use a raw_input(), the input will be a string, which we have to cast into an integer. If the input is not a valid integer then exception will generate (raise) a ValueError.

Try and Catch: First we will use try if there is any error in a python program and after then we will use catch and at last we will use exception.
Finally: Finally is used every time before leaving the try statement even if exception occurs or not. The error will be re-raised when final clause is executed only if the exception occured in try which is not handled by except clause.
Example:

inputVal = int(raw_input("Please enter a number: "))
Please enter a number: 34.5
ValueError: invalid literal for int() with base 10: '34.5'
File "C:\Personal\PythonScript\python-ppt\TechTra\setExample.py", line 39, in 
inputVal = int(raw_input("Please enter a number: "))
With the aid of exception handling, we can write robust code for reading an integer from input:
while True: 
try: inputVal = raw_input("Please enter an integer: ") 
inputVal = int(inputVal ) 
break 
except ValueError: 
print("No valid integer! Please try again ...") 
print "Great, you successfully entered an integer!"