Custom Exceptions

Custom exceptions allow you to create your own error types.
They are useful when built-in exceptions do not clearly describe your program’s specific error.

Custom exceptions make your code more readable, professional, and organized.

WHY CREATE CUSTOM EXCEPTIONS?

• Enforce business rules
• Improve error clarity
• Make debugging easier
• Handle application-specific problems

CREATING A CUSTOM EXCEPTION

To create a custom exception, define a class that inherits from Exception.

class CustomError(Exception):
    pass

Now you can use this exception in your program.

SIMPLE EXAMPLE

class InvalidAgeError(Exception):
    passage = int(input("Enter your age: "))if age < 0:
    raise InvalidAgeError("Age cannot be negative.")print("Age accepted.")

If the user enters a negative age, the custom exception will be raised.

USING CUSTOM EXCEPTION WITH TRY-EXCEPT

class InvalidMarksError(Exception):
    passtry:
    marks = int(input("Enter marks: "))
    if marks < 0 or marks > 100:
        raise InvalidMarksError("Marks must be between 0 and 100.")
except InvalidMarksError as e:
    print("Error:", e)

Here, the custom exception is raised and handled properly.

ADDING CUSTOM MESSAGE WITH INIT

You can customize the exception class further.

class NegativeNumberError(Exception):
    def __init__(self, message):
        self.message = message
        super().__init__(self.message)try:
    num = int(input("Enter number: "))
    if num < 0:
        raise NegativeNumberError("Negative numbers are not allowed.")
except NegativeNumberError as e:
    print("Error:", e)

REAL-WORLD USE CASE

Example: Bank account balance validation

class InsufficientBalanceError(Exception):
    passbalance = 500
withdraw = 1000if withdraw > balance:
    raise InsufficientBalanceError("Not enough balance.")

This makes your application logic clearer and more structured.

BEST PRACTICES

• Inherit from Exception
• Use meaningful class names
• Provide clear error messages
• Use custom exceptions for specific business logic

KEY TAKEAWAY

Custom exceptions allow you to define and handle application-specific errors clearly and professionally, making your Python programs stronger and easier to maintain.