8 5 Custom Exceptions Explained

Key Concepts

Custom exceptions in Python allow you to define your own exception classes. This is useful for creating application-specific error conditions. The key concepts include:

• Defining Custom Exceptions
• Raising Custom Exceptions
• Handling Custom Exceptions
• Inheritance in Custom Exceptions
• Practical Applications

1. Defining Custom Exceptions

To create a custom exception, you define a new class that inherits from the built-in Exception class or any of its subclasses.

Example:

class CustomError(Exception):
    pass
    

Analogy: Think of creating a new rule in a game that triggers a specific penalty.

2. Raising Custom Exceptions

You can raise custom exceptions using the raise statement. This allows you to signal that a specific error condition has occurred.

Example:

def check_value(value):
    if value < 0:
        raise CustomError("Value cannot be negative!")

try:
    check_value(-5)
except CustomError as e:
    print(e)
    

Analogy: Think of a referee signaling a foul in a game.

3. Handling Custom Exceptions

Handling custom exceptions is similar to handling built-in exceptions. You use a try-except block to catch and handle the custom exception.

Example:

try:
    check_value(-5)
except CustomError as e:
    print(e)
    

Analogy: Think of a safety net in a circus act. If the performer falls, the net catches them.

4. Inheritance in Custom Exceptions

Custom exceptions can inherit from other custom exceptions, allowing for more granular error handling. This is useful for organizing exceptions into a hierarchy.

Example:

class NegativeValueError(CustomError):
    pass

def check_value(value):
    if value < 0:
        raise NegativeValueError("Value cannot be negative!")

try:
    check_value(-5)
except NegativeValueError as e:
    print(e)
    

Analogy: Think of a multi-layered safety net, each layer designed to catch a different type of fall.

5. Practical Applications

Custom exceptions are useful in various scenarios, such as validating user input, handling business logic errors, and more.

Example:

class InvalidInputError(CustomError):
    pass

def validate_input(user_input):
    if not user_input.isdigit():
        raise InvalidInputError("Input must be a digit!")

try:
    validate_input("abc")
except InvalidInputError as e:
    print(e)
    

Analogy: Think of creating a new rule in a game that triggers a specific penalty.

Putting It All Together

By understanding and using custom exceptions effectively, you can create more robust and maintainable Python applications.

Example:

class CustomError(Exception):
    pass

class NegativeValueError(CustomError):
    pass

def check_value(value):
    if value < 0:
        raise NegativeValueError("Value cannot be negative!")

try:
    check_value(-5)
except NegativeValueError as e:
    print(e)