Python Tutorial - Exception Handling

1. try...except
2. raise Exception
3. try...finally
4. Python Built-In Exceptions

You will learn how to handle Python exceptions in this section. Further, you will learn to raise and catch exceptions.

When an exception occurs in a program, the program execution is terminated. But most of the time, you don’t prefer the abnormal termination of the program, therefore you could use Python exceptions handling to avoid the crash of your program.

try...except

In the following example, the user is asked to enter 10 integers. If the user enters another type rather than integers, an exception will be thrown and statements in except block is executed:

>>> n = 10
>>> i = 1
>>> sum = 0
>>> while i <= n:
	try:
		num = int(input('Enter a number: '))
		sum = sum + num
		i = i+1
	except:
		print('Please enter an integer!!')
		break

	
Enter a number: 23
Enter a number: 12
Enter a number: Asd
Please enter an integer!!
>>> print('Sum of numbers =', sum)
Sum of numbers = 35

You can also catch specified exceptions, for example, if division by zero happens, you can catch the corresponding exception as:

try:
    # statements
except ZeroDivisionError:
    # exception handling
except:
    # all other exceptions are handled here

raise Exception

You can force a specific exception to occur by using raise keyword:

>>> raise ZeroDivisionError
Traceback (most recent call last):
  File "<pyshell#17>", line 1, in <module>
    raise ZeroDivisionError
ZeroDivisionError

>>> raise ValueError
Traceback (most recent call last):
  File "<pyshell#18>", line 1, in <module>
    raise ValueError
ValueError

If you need to add some customized information to the self-raised exception, you could add it in the parenthesis after exception error type.

>>> raise NameError("This is a customized exception")
Traceback (most recent call last):
  File "<pyshell#18>", line 1, in <module>
    raise NameError("This is a customized exception")
NameError: This is a customized exception

try...finally

The finally block is optional in the try block. If there is a finally block, it will be executed in any case.

This is demonstrated in the example below:

>>> try:
		raise ValueError
finally:
		print('You are in finally block')

	
You are in finally block
Traceback (most recent call last):
  File "<pyshell#23>", line 2, in <module>
    raise ValueError
ValueError

The try...finally block is used for the clean-up actions such as when working with a file, it is a good practice to close the file. So the file will be closed in the finally block.

Python Built-In Exceptions

In this section, you will learn about the standard exceptions in Python programming.

Exceptions are the runtime errors that are raised during interpretation. For example, the division of any number by zero will create a runtime error called ZeroDivisionError.

In Python, there are some built-in exceptions that are described below: