Raising Exceptions

In Python, you can raise exceptions manually using the “raise” keyword. This allows you to handle error cases in your code in a specific way and can be a useful tool for writing more robust and reliable code.

To raise an exception in Python, you use the “raise” keyword followed by the exception type and a message. For example:

raise ValueError("Invalid input")

In the above example, a ValueError exception is raised with the message “Invalid input”.

Define Custom Exceptions

You can also define your own custom exception types by creating a new class that inherits from the “Exception” class. For example:

class CustomException(Exception):
    pass

raise CustomException("A custom exception has occurred.")

In the above example, a new exception type called “CustomException” is defined and a CustomException exception is raised with the message “A custom exception has occurred.”

It’s important to note that exceptions should only be raised when there is a genuine error or exceptional condition that cannot be handled in any other way. Overuse of exceptions can make your code more difficult to understand and maintain.

Here is an example of how you might use exceptions to handle error cases in a function that calculates the mean of a list of numbers:

def calculate_mean(numbers):
    if not numbers:
        raise ValueError("Cannot calculate mean of an empty list.")
    return sum(numbers) / len(numbers)

try:
    result = calculate_mean([])
except ValueError as e:
    print(e)

In the above example, the calculate_mean() function checks if the list of numbers is empty, and if it is, it raises a ValueError with the message “Cannot calculate mean of an empty list.” The try and except block handles the ValueError and prints the error message.

Using exceptions in this way allows you to handle error cases in a controlled manner and provide more informative messages to the user or calling code.

Conclusion

Overall, raising exceptions is a useful tool for handling errors and exceptional conditions in Python, and can help you write more robust and reliable code.

Exercises

Here are some exercises with solutions to help you practice what you just learned:

raise ValueError("Invalid input")

class CustomException(Exception):
    pass

raise CustomException("A custom exception has occurred.")

try:
    # code that may cause an exception
except ExceptionType:
    # code to handle the exception