Custom Exceptions in Python — Creating Custom Exceptions (59/100 Days of Python)

Day 59 of the “100 Days of Python” blog post series covering custom exceptions

In Python, exceptions are used to signal errors or unexpected events that occur during program execution. Python has a set of built-in exceptions, such as SyntaxError, TypeError, and ValueError. However, sometimes these built-in exceptions may not be sufficient for your needs, and you may need to define your own custom exceptions. In this tutorial blog post, we will explore how to create custom exceptions in Python and why they are useful.

Why Create Custom Exceptions?

Custom exceptions are helpful in many situations. They allow you to define your own error conditions and handle them in a more specific and meaningful way. For example, let’s say you are writing a program that reads data from a file. If the file is not found, Python will raise a FileNotFoundError. However, this exception may not provide enough information to the user about what went wrong. In this case, you can define your own custom exception, such as FileNotFoundCustomError, which can provide more specific details about the error.

Custom exceptions are also useful in object-oriented programming, where you may want to define exceptions that are specific to your classes. For example, if you are creating a banking application, you may want to define an exception that is raised when a user tries to withdraw more money than they have in their account. This exception can be defined as a custom exception, such as InsufficientFundsError, which makes it easier to handle this specific error condition.

Creating Custom Exceptions in Python

Creating custom exceptions in Python is easy. You can create a custom exception by creating a new class that inherits from the built-in Exception class:

class CustomException(Exception):
    pass

In the above code, we have defined a new custom exception called CustomException, which inherits from the built-in Exception class. The pass statement indicates that we are not adding any additional functionality to the exception at this time.

To raise this exception in your code, you can simply use the raise keyword followed by an instance of the custom exception:

raise CustomException('This is a custom exception')

In the above code, we are raising an instance of our custom exception CustomException with the message "This is a custom exception".

Adding Functionality to Custom Exceptions

In some cases, you may want to add additional functionality to your custom exceptions. For example, you may want to add an error code or additional data to the exception object. To do this, you can define a new constructor for your custom exception and use the super() function to call the constructor of the base Exception class:

class CustomException(Exception):
    def __init__(self, message, error_code):
        super().__init__(message)
        self.error_code = error_code

In the above code, we have defined a new constructor for our custom exception that takes two arguments: message and error_code. We use the super() function to call the constructor of the base Exception class and pass the message argument to it. We then set the error_code attribute to the error_code argument.

To raise this exception in your code, you can now pass both the message and error code to the constructor, like this:

raise CustomException('This is a custom exception', 1234)

Best Practices for Naming Custom Exceptions in Python

Naming exceptions in Python is an important aspect of creating custom exceptions. Good naming practices can make it easier to understand the purpose of the exception, while bad naming practices can lead to confusion and make debugging more difficult.

Use descriptive names

When naming exceptions, it is important to use descriptive names that clearly describe the error or condition being raised. A good exception name should provide enough information to the reader to understand what went wrong without having to read the code.

For example, instead of using a generic name like CustomException, use a more descriptive name that explains what the exception is for, such as FileNotFoundError or InvalidParameterError.

End exception names with “Error”

In Python, it is common to end exception names with the word “Error”. This makes it clear that the class is an exception and helps to distinguish it from other classes.

For example, if you have a custom exception for handling network errors, you could name it NetworkError.

Avoid abbreviations

Abbreviations should generally be avoided when naming exceptions. Abbreviations can make it difficult for others to understand the meaning of the exception, especially if the abbreviation is not commonly known.

For example, instead of naming an exception InvldParamErr, use the more descriptive InvalidParameterError.

Use Built-in Exceptions When Possible

Python has a set of built-in exceptions that cover many common error conditions. When creating custom exceptions, it is important to choose a specific exception type that best describes the error being raised.

For example, if you are raising an exception because a file could not be found, you should use the built-in FileNotFoundError exception instead of creating a custom exception.

What’s next?

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