Adventures in Machine Learning

Mastering Python Exceptions: Handling and Raising Custom Errors

Have you ever encountered an error message when working with Python code that you couldn’t quite understand? Perhaps you received a TypeError message, indicating that one of your variables was of the wrong data type.

Or maybe you received an IndexError message, telling you that you were trying to access an index that did not exist. These are just examples of the many types of exceptions that can occur when working with Python code.

In this article, we will explore how Python handles exceptions and how to manually raise them when needed. We will also discuss creating custom exceptions with custom error messages, which can be especially useful when developing APIs or libraries.

Raising Exceptions in Python

When Python code encounters an error, it will typically raise what is known as an exception. Exceptions can signal a variety of issues, such as syntax errors, data type errors, or compatibility issues.

Understanding how to read and handle these exceptions can help make your code more robust and better prepared for a variety of scenarios.

Types of Exceptions in Python

Python has several built-in types of exceptions that can be raised in response to different issues. Some of the most common include:

– TypeError: This occurs when an operation or function is applied to an object of inappropriate type.

– ValueError: This occurs when a function is passed an argument of the correct type, but with an inappropriate value. – IndexError: This occurs when trying to access an index that does not exist in a list or sequence.

– KeyError: This occurs when trying to access a key in a dictionary that does not exist. – NameError: This occurs when trying to access a variable that has not been defined.

Manually Raising Exceptions

Aside from built-in exceptions, Python also allows us to manually raise exceptions when needed. This can be useful when creating more complex code and you want to handle specific issues with custom error messages.

To manually raise an exception, we use the ‘raise’ keyword followed by an exception type, such as ValueError or TypeError. For example, suppose we have a function that accepts only integer values.

We can use a custom error message to make it clear to users when they try to input the wrong data type. “`python

def divide(x, y):

if type(x) != int or type(y) != int:

raise TypeError(“Arguments must be integers”)

return x/y


In the example above, we check if both arguments are integers.

If not, we raise a custom TypeError message indicating that only integers are allowed.

Defining Custom Exceptions in Python

While built-in exceptions cover most scenarios, there may be times when we need to create custom exceptions to handle specific issues. This can be especially useful when developing APIs or libraries and you want to provide more specific error messages to users.

Creating a Custom Exception Class

To create a custom exception class, we simply need to define a new class that inherits from the base Exception class. This new class can then be used to raise exceptions when needed.


class MyException(Exception):



In the example above, we define a custom exception class called ‘MyException’. We inherit from the base Exception class and don’t add any additional functionality.

However, in a real-world scenario, we might add additional properties or methods to the class to provide more specific information about the error.

Adding Custom Messages and Errors

To add custom messages and errors to our custom exception class, we can define the ‘__str__’ method. This method will be called whenever we try to convert our exception to a string, allowing us to provide a custom message for the error.


class MyException(Exception):

def __str__(self):

return “Something went wrong.”


In the example above, we define the ‘__str__’ method for our custom exception class. Whenever this exception is raised, the custom message “Something went wrong.” will be displayed.


In conclusion, exceptions are a key part of Python programming and understanding how to handle and raise them is essential to writing robust and error-free code. Whether using built-in exceptions or creating custom ones, being able to provide clear error messages can make debugging and troubleshooting much easier.

By following the steps outlined in this article, you can start creating more reliable and maintainable Python code. Python exceptions can signal a variety of issues in code such as data type errors, code compatibility and syntax errors.

Manually raising exceptions can help create more complex codes and custom error messages. Built-in exceptions include common exceptions such as TypeError, ValueError, and IndexError.

Its also possible to create a custom exception class by defining a class that inherits from an imported Exception class. Overall, understanding how to handle and raise exceptions can improve the quality of Python code by making it more reliable and maintainable.

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