🚀 Day 91/150 – Custom Exceptions in Python

Python provides many built-in exceptions like ValueError, TypeError, and ZeroDivisionError. But sometimes you may need to create your own exception to represent specific errors in your program. This is called a custom exception.

In this post, we'll learn four ways to work with custom exceptions in Python.

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Method 1 – Creating a Basic Custom Exception

Create your own exception by inheriting from the built-in Exception class.

class AgeError(Exception):
    pass

age = int(input("Enter your age: "))

if age < 18:
    raise AgeError("You must be at least 18 years old.")

print("Access Granted!")

Sample Input

16

Output

AgeError: You must be at least 18 years old.

Explanation

• AgeError is a custom exception class.
• raise is used to manually trigger the exception.
• If the age is less than 18, Python raises AgeError.

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Method 2 – Handling a Custom Exception

You can catch your custom exception using try and except.


class AgeError(Exception):

    pass

try:
    age = int(input("Enter your age: "))

    if age < 18:
        raise AgeError("You must be at least 18 years old.")

    print("Access Granted!")

except AgeError as error:
    print(error)












Sample Input

15

Output

You must be at least 18 years old.

Explanation

• The custom exception is raised inside the try block.
• The except block catches AgeError.
• The program continues instead of crashing.

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Method 3 – Custom Exception with a Custom Message

You can define your own message inside the exception class.

class NegativeNumberError(Exception):
    def __init__(self):
        super().__init__("Negative numbers are not allowed.")

num = int(input("Enter a number: "))

if num < 0:
    raise NegativeNumberError()

print("Valid Number")









Sample Input

-8

Output

Negative numbers are not allowed.

Explanation

• The constructor (__init__) defines a default error message.
• Whenever the exception is raised, the message is displayed automatically.

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Method 4 – Custom Exception in a Function

Custom exceptions are commonly used inside functions.

class PasswordError(Exception):
    pass

def check_password(password):
    if len(password) < 8:
        raise PasswordError("Password must contain at least 8 characters.")

    return "Password Accepted"

try:
    print(check_password(input("Enter password: ")))

except PasswordError as error:
    print(error)

Sample Input

python

Output

Password must contain at least 8 characters.

Explanation

• The function checks the password length.
• If the password is too short, it raises a custom exception.
• The exception is caught outside the function using try and except.

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Comparison of Methods

Method	Best Used For
Basic Custom Exception	Creating your own exception type
Custom Exception with try/except	Handling custom errors gracefully
Custom Message	Providing meaningful error messages
Function-based Exception	Validating function inputs

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🔥 Key Takeaways

• A custom exception is a user-defined exception created by inheriting from the Exception class.
• Use the raise keyword to trigger a custom exception.
• Catch custom exceptions using try and except.
• Custom exceptions make your programs easier to understand and debug.
• They are useful for validating user input and enforcing application-specific rules.
• Giving custom exceptions meaningful names and messages makes your code more readable.