Table of Contents

Python User-Defined Exceptions

Python has many built-in exceptions which force the program to raise an error when something goes wrong. However, sometimes, we may need to create custom exceptions that serve our purpose.

In Python, users can define custom exceptions by creating a new class. This exception class must be derived directly or indirectly from the Exception class, which is the source of most built-in exceptions.

Example

Let’s understand the concept with an example. In this example, we will illustrate how user-defined exceptions can be used in a program to raise and catch errors. This program will prompt the user to enter a number until they guess a stored number correctly. To help them figure it out, a hint is provided as to whether their guess is greater than or less than the secret.

Python Program

########### # Python program User defined Exception # Python Tutorials - www.TestingDocs.com ###########

class GuessSmallError(Exception):
“Exception raised when guess is small”
pass

class GuessLargeError(Exception):
“Exception raised when guess is large”
pass

# Secret number .. ssshhhh!!!
secret = 21

# Python while loop
while True:
#Python try-except code block
try:
guess= int(input(“Enter guess number:”))
if(guess < secret): raise GuessSmallError elif(guess > secret):
raise GuessLargeError
else:
break
except GuessSmallError:
print(“Your guess is small, Try Again…”)
except GuessLargeError:

print(“Your guess is large, Try Again…”)

print(“Congratulations! Your guess is correct.”)

Python User Defined Exceptions

Sample Program Run Output

= RESTART: C:/PythonPrograms/user_defined.py
Enter guess number:45
Your guess is large, Try Again…
Enter guess number:3
Your guess is small, Try Again…
Enter guess number:10
Your guess is small, Try Again…
Enter guess number:17
Your guess is small, Try Again…
Enter guess number:22
Your guess is large, Try Again…
Enter guess number:19
Your guess is small, Try Again…
Enter guess number:20
Your guess is small, Try Again…
Enter guess number:21
Congratulations! Your guess is correct.