Custom Exceptions in Python

Learn how to create your own exception classes in Python for application-specific errors.

What are Custom Exceptions?

Python provides many built-in exceptions such as ValueError, TypeError and ZeroDivisionError.

Sometimes an application needs an error type that is specific to its own business rules or requirements. In that situation, we can create a custom exception.

Custom Exception Flow

Basic Syntax

class MyException(Exception):

    pass

A custom exception class normally inherits from the built-in Exception class.

Simple Example

class AgeError(Exception):

    pass


try:

    age = 15

    if age < 18:

        raise AgeError("Age must be 18 or above")

except AgeError as error:

    print(error)

Output:

Age must be 18 or above

Here, AgeError is a custom exception created by the programmer.

Custom Exception with User Input

class AgeError(Exception):

    pass


try:

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

    if age < 18:

        raise AgeError("You must be 18 or above")

    print("Valid age")

except AgeError as error:

    print("Error:", error)

Example Output:

Enter your age: 16
Error: You must be 18 or above

Example: Marks Validation

class InvalidMarksError(Exception):

    pass


try:

    marks = int(
        input("Enter marks: ")
    )

    if marks < 0 or marks > 100:

        raise InvalidMarksError(
            "Marks must be between 0 and 100"
        )

    print("Valid marks:", marks)

except InvalidMarksError as error:

    print("Error:", error)

Example Output:

Enter marks: 120
Error: Marks must be between 0 and 100

Custom Exception with a Value

A custom exception can also store information about the error.

class SalaryError(Exception):

    def __init__(self, salary):

        self.salary = salary


try:

    salary = 5000

    if salary < 10000:

        raise SalaryError(salary)

except SalaryError as error:

    print("Invalid salary:", error.salary)

Output:

Invalid salary: 5000

Multiple Custom Exceptions

A program can contain more than one custom exception class for different business rules.

class AgeError(Exception):

    pass


class MarksError(Exception):

    pass


try:

    age = 16

    marks = 120

    if age < 18:

        raise AgeError("Age must be 18 or above")

    if marks < 0 or marks > 100:

        raise MarksError("Invalid marks")

except AgeError as error:

    print(error)

except MarksError as error:

    print(error)

Each custom exception can have its own handler.

Custom Exception with Function

class InsufficientBalanceError(Exception):

    pass


def withdraw(balance, amount):

    if amount > balance:

        raise InsufficientBalanceError(
            "Insufficient balance"
        )

    return balance - amount


try:

    balance = 5000

    amount = 7000

    remaining = withdraw(balance, amount)

    print("Remaining balance:", remaining)

except InsufficientBalanceError as error:

    print("Error:", error)

Output:

Error: Insufficient balance

Practical Uses of Custom Exceptions

Built-in vs Custom Exceptions

Best Practices

Common Mistakes

• Creating unnecessary custom exceptions.
• Using unclear exception names.
• Not providing a meaningful error message.
• Forgetting to inherit from the Exception class.

Practice Programs

1. Create a custom exception for invalid age.
2. Create a custom exception for invalid marks.
3. Create a custom exception for insufficient balance.
4. Create two custom exceptions for two different application rules.
5. Use a custom exception inside a function.