Custom Exceptions

Custom Exceptions in Python

In addition to built-in exceptions, Python allows you to create your own custom exceptions. Custom exceptions are used when you need to define specific error conditions in your code that aren't covered by the standard exceptions. This section explains how to create and use custom exceptions.

1. Creating Custom Exceptions

To create a custom exception, you need to define a new class that inherits from the built-in Exception class or one of its subclasses. Here's an example:

class CustomError(Exception):
    def __init__(self, message):
        self.message = message

In the above code, we define a custom exception called CustomError, which inherits from the base Exception class. We also provide an __init__ method to set an error message.

2. Raising Custom Exceptions

You can raise your custom exception using the raise statement. For example:

def some_function():
    # Check a condition
    if condition_is_met:
        raise CustomError("This is a custom exception.")

3. Handling Custom Exceptions

You can handle custom exceptions just like built-in exceptions using the try...except block. Here's an example:

try:
    some_function()
except CustomError as e:
    print(f"Custom exception caught: {e}")

4. When to Use Custom Exceptions

Custom exceptions are useful when:

• Your code has specific error conditions that aren't adequately covered by standard exceptions.
• You want to make your code more readable and maintainable by providing clear error messages for custom situations.
• You need to create a hierarchy of related exceptions for your application.

5. Exception Hierarchies

You can create hierarchies of custom exceptions by defining multiple exception classes that inherit from each other. This allows you to capture different levels of specificity in your error conditions.

For example:

class BaseCustomError(Exception):
    pass

class SpecificCustomError(BaseCustomError):
    pass

class VerySpecificCustomError(SpecificCustomError):
    pass

6. Best Practices

When creating custom exceptions, consider the following best practices:

• Name your custom exceptions descriptively to convey their purpose.
• Document your custom exceptions and their use cases.
• Avoid defining too many custom exceptions; use them judiciously.
• Make sure your custom exceptions provide clear and informative error messages.

Custom exceptions can improve the clarity and reliability of your code by allowing you to handle specific error conditions with precision. They are particularly valuable in larger and more complex codebases.