Python’s built-in exception classes do not address certain error situations that may arise in your code. In such cases, you’ll need to create custom exceptions to handle these errors effectively.
In Python, you can define custom exceptions and raise them when specific error situations occur. You can manage specific, informative errors with custom exceptions, improving your code’s readability and maintainability.
Why Do You Need Custom Exceptions?
During the development of an application, various error scenarios can arise due to changes in the code, integration with other packages or libraries, and interactions with external apps. It is crucial to handle these errors to recover from them or handle failure gracefully.
Python offers a range ofbuilt-in exceptionclasses that cover errors such asValueError,TypeError,FileNotFoundError, and more. While these built-in exceptions serve their purpose well, they may only sometimes accurately represent the errors that can occur in your application.
By creating custom exceptions, you can tailor them specifically to suit the requirements of your application and provide information for developers who utilize your code.
How to Define Custom Exceptions
To create custom exceptions,define a Python classthat inherits from theException class. TheExceptionclass offers base functionality you’ll need to handle exceptions, and you can customize it to add features based on your specific needs.
When creating custom exception classes, keep them simple while including necessary attributes for storing error information. Exception handlers can then access these attributes to handle errors appropriately.
Here’s a custom exception class,MyCustomError:
This class accepts an optional message argument during initialization. It uses thesuper()method to call the constructor of the baseExceptionclass, which is essential for exception handling.
How to Raise Custom Exceptions
To raise an error, use theraisekeyword followed by an instance of your custom exception class, passing it an error message as an argument:
You can also raise the error without passing any arguments:
Either format is suitable for raising custom errors.
How to Handle Custom Exceptions
Handling custom exceptions follows the same approach ashandling built-in exceptions. Usetry,except, andfinallyblocks to catch custom exceptions and take appropriate action.
This way, you can handle all forms of custom exceptions raised.
If an exception occurs during the execution of atryblock, a correspondingexceptblock can catch and handle it. If there is no appropriateexceptblock to handle the exception, anyfinallyblock will execute, followed by the exception raising again. Use afinallyblock primarily to perform clean-up tasks that must run in any circumstances, whether an exception occurs or not.
In this sample, aKeyboardInterruptexception occurs, but theexceptblock only handlesMyCustomErrorexceptions. In this case, thefinallyblock runs, and then the unhandled exception raises again.
Inheriting Custom Error Classes
Based on theconcept of Object-Oriented Programming(OOP), you can also inherit from custom exception classes, just like regular classes. By inheriting from a custom exception class, you can create error classes that provide more specific context to an exception. This approach allows you to handle errors at different levels in your code and provides a better understanding of what caused the error.
Say you’re developing a web application that interacts with an external API. This API might have different error scenarios. You’ll want to handle these errors consistently and clearly throughout your code. To achieve this, create a custom exception class,BaseAPIException:
Once you have this base custom exception class, you can create child exception classes that inherit from it:
Raise and catch these custom exceptions when making calls to the API within your web application. Handle them accordingly using the appropriate logic in your code.
The final except clause checks against the parent class and acts as a catch-all for any other API-related errors.
When you inherit custom exception classes, you can effectively handle errors within the API. This approach allows you to separate your error handling from the API’s implementation details, making it easier to add custom exceptions or make changes as the API evolves or encounters new error cases.
Wrapping Custom Exceptions
To wrap exceptions means to catch an exception, encapsulate it within a custom exception, and then raise that custom exception while referencing the original exception as its cause. This technique helps provide context to error messages and keeps implementation details hidden from the calling code.
Consider the scenario where your web app interacts with an API. If the API raises aLookupError, you can catch it, then raise a customAPINotFoundErrorexception that references the LookupError as its cause:
Use afromclause with theraisestatement to reference the original exception within your custom exception.
When the custom exception occurs, it includes the original exception as a__cause__attribute, providing a link between the custom exception and the original. This lets you trace the origin of an exception.
By wrapping exceptions, you can provide more meaningful context and send more appropriate error messages to users, without revealing internal implementation details of your code or the API. It also lets you manage and address types of errors in a structured and uniform way.
Customizing Class Behavior in Python
By inheriting the base exception class that Python provides, you may create simple and useful exceptions that you can raise when specific errors occur in your code. You can also implement custom behavior for your exception classes with the help of magic or dunder methods.
