This document discusses exception handling in Python. It explains that exceptions occur when runtime errors happen and stop the normal flow of a program. Exceptions can be caused by syntax errors or errors in code that result in an exception even if the syntax is valid, like division by zero. The try and except statement is used to handle exceptions - critical code is placed in the try block and exception handling code is placed in the except block. Multiple except blocks can handle different exceptions. An else block will run only if no exceptions occurred. Finally, a finally block is always executed after try and except to clean up resources. Exceptions can also be manually raised using raise or re-raised after handling.

1. EXCEPTION HANDLING IN
PYTHON
Dr. V.Sireesha
Dept. of CSE

2. Exception
• When runtime error occurs in program, it creates an exception
• When exception occurs ,
• Python interpreter will stop the current process
• Pass the exception to the calling process until it is handled
• If not handled , an error message is displayed and program comes to a
sudden unexpected halt
• Error in Python can be of two types
• Syntax errors –due to wrong syntax
• Exceptions- program is syntactically correct but the code resulted in an
error.

3. Exception
• Example for syntax error
• Example for exception
• Division by zero
• Accessing a non existing element in the list

4. Exception
• Accessing a key which is not present in dictionary
• Opening a non existing file

5. Exception Handling

6. Exception handling-using try and except
• Using try and except statement
• The critical operations which can raise an exception will be placed in try
clause
• The code that handles the exception will be placed in except block

7. Multiple except clause
• We an also use multiple except block to handle different
exception
• Please note that at most one handler will be executed.

8. Else clause
• In Python we can use an else clause on the try –except block
• The else clause must be present after all the except clause
• The code enters the else block only if the try clause does not
raise any exception

9. Finally keyword in Python
• Python provides a keyword finally, which is always executed
after try and except blocks.
• Generally used to release external resources.
• We must clean up the resource before the program comes to a
halt whether it successfully ran or not.
• These actions (closing a file, GUI or disconnecting from
network) are performed in the finally clause

10. Finally keyword in Python

11. Exception Handling

12. Raise an exception-raise
• We can also manually raise exceptions using the raise
keyword.

13. Raise an exception-raise

14. except Exception as e:
• if you want to print the message like what is an error in a
program then we use “e” which is the representation or object
of an exception.
• try:
print(5/0)
except Exception as e:
print("Error",e)
Output:
Error division by zero

17. Raising Exceptions

18. re-raise an exception

19. Instantiating an exception

20. Raise an exception with arguments

21. Handling Exceptions in invoked functions

22. Handling Exceptions in invoked functions

23. Handling Exceptions in invoked functions

24. Nested try

25. re-raise the exception

26. Assert

27. Assert