1. ADVANCE COMPUTER
PROGRAMMING
Week 9
By: Ammarah Khalid (Lecturer CS)
Riphah International University (Faisalabad)

2. Exceptions
• An exception is a condition which occurs during the
execution of a program that disrupts the normal flow of
the program's instructions
• In general, when a Python script encounters a situation
that it cannot cope with, it raises an exception
• Whenever an exception occurs, the program stops the execution,
and thus the further code is not executed
• An exception is a Python object that represents an
runtime error

3. Exceptions (Cont.)

4. Common Exceptions
• Python provides the number of built-in exceptions
• A list of common exceptions that can be thrown from a
standard Python:
• ZeroDivisionError: It occurs when a number is divided by zero
• NameError: It occurs when a name is not found. It may be local or
global
• ValueError: It occurs when there is an issue with the content of
the object to which you wanted to assign a value
• IOError: It occurs when Input Output operation fails
• EOFError: It occurs when the end of the file is reached, and yet
operations are being performed

5. If Exception Is Not Handled..

6. Exception Handling In Python

7. try/except
• To handle exceptions we need to use try and except block
• Define risky code that can raise an exception inside
the try block and corresponding handling code inside
the except block

8. try/except Syntax And Example

9. Using else With try/except
• Use the else statement with the try-except statement in
case to address the code which will be executed if no
exception occurs in the try block

10. Using else With try/except (Cont.)

11. Using else With try/except (Cont.)

12. Using else With try/except (Cont.)
• Exception variable can also be used with except
statement
• It is done by using the as keyword
• This object will return the cause of the exception

13. Catching Specific Exception
• A specific exception can also be caught
• It is good practice to specify an exact exception that the
except clause should catch
• Suppose the user enters the denominator as zero
• In that case, the try block will throw a ZeroDivisionError,
and we can catch that

14. Handling Multiple Exceptions
• A piece of code can throw several different exceptions
• It is needed to account for all of the potential exceptions
that could be raised within try block
• If the user enters a non-numeric value then, the try block
will throw a ValueError exception

15. Handling Multiple Exceptions (Cont.)
• If the user enters the denominator as zero, the try block
will throw a ZeroDivisionError
• Thus it can be specified which exception except block
should catch or handle
• A try block can be followed by multiple numbers
of except blocks to handle the different exceptions
• But only one exception will be executed when an
exception occurs

16. Handling Multiple Exceptions (Cont.)
• Multiple exceptions can also be handled with a
single except clause
• For that, use an tuple of values to specify multiple
exceptions in an except clause

17. try/finally
• Python provides the finally block, which is used with the
try block statement
• The finally block is used to write a block of code that
must execute, whether the try block raises an
exception or not
• Mostly, the finally block is used to release the external
resource
• This block provides a guarantee of execution

18. try/finally (Cont.)

19. try/finally (Cont.)

20. raise
• In Python, the raise statement allows to throw an
exception manually
• The single argument in the raise statement is an
exception to be raised
• It can be either an exception object or an Exception class
that is derived from the Exception class
• The raise statement is useful in situations where we need
to raise an exception explicitly
• For example, we can raise exceptions in cases such as
wrong data received or any validation failure

21. Steps To raise An Exception
• Give an exception of the appropriate type
• Use the existing built-in exceptions or create your own exception as
per the requirement
• Pass the appropriate data while raising an exception
• Execute a raise statement, by providing the exception
class
• Syntax:

22. raise (Example)

23. Custom Exceptions
• In Python, users can define custom exceptions by
creating a new class
• This exception class has to be derived from the built-
in Exception class
• Most of the built-in exceptions are also derived from this
class

24. Custom Exceptions (Example 1)

25. Custom Exceptions (Example 2)

26. Custom Exceptions (Example 3)