Manually Raise or Throw an Exception in Python

You can use the ‘raise’ keyword to manually raise an exception in Python.

Raising an exception is important in Python because it is used to signal an error or an unexpected condition. In this blog, let’s explore the different methods for raising exceptions in  Python.

Table of Contents:

• What is an exception in Python?
• Methods to throw an exception by hand in Python:
  • Method 1: Throwing an exception in Python Using the raise Keyword
  • Method 2: Raising Built-in Exceptions in Python
  • Method 3: Using raise with a specific error message in Python
  • Method 4: Using Custom Exceptions in Python
  • Method 5: Raising an exception using an Assert Statement in Python
  • Method 6: Raising an exception using sys.exc_info() in Python
• Best Practices for Raising Exceptions in Python
• Exceptions vs Syntax Errors in Python
• Conclusion

What is an Exception in Python?

An exception in Python is a signal that occurs during the execution and interrupts the flow of the program. It can occur when divided by zero or if any file is missing. Python provides a try-except to catch and fix errors. Handling exceptions helps in specifying the errors without crashing the program.

Methods to Manually Raise or Throw an Exception in Python

Python provides various approaches for raising exceptions. Here are some methods. 

Method 1: Throwing an Exception in Python Using the raise Keyword

The basic way to raise an exception is through the keyword raise. This is a general method of manually raising an exception. It helps stop the program when a certain condition is met by raising an exception.

Example:

Output:

Explanation: Here, when the condition for raising the exception is met, Python raises an error and prints the message that is provided.

Method 2: Raising Built-in Exceptions in Python

Python offers numerous built-in exceptions, such as IndexError, TypeError, and KeyError. You can use these exceptions manually based on the error.

Example:

Output:

Explanation: Here, we are using a built-in exception IndexError to display that the index variable was out of range in some list or sequence

Method 3: Using Raise with a Specific Error Message in Python

It is useful to add extra information to the exception message, which can be done very easily with the raise keyword. This is very useful for debugging.

Example:

Output:

Explanation: Here, we raise a ValueError whenever the age is invalid (negative), along with an informative message indicating why it failed.

Method 4: Using Custom Exceptions in Python

In some cases, you may also want to have specific exceptions for particular errors. You do this by first importing the Exception class, and then deriving the exception.

Example:

Output:

Explanation: Here, we are raising a value error every time the age is invalid (less than zero), and with a message that explains why it has failed.

Method 5: Raising an Exception Using an Assert Statement in Python

The assert statement ensures a condition is met. If the condition is False, it raises an AssertionError.

Example:

Output:

Explanation: Here the assert statement checks if the age is 18 or more. If not, it raises an AssertionError.

Method 6: Raising an Exception Using sys.exc_info() in Python 

The sys.exc_info() function captures the exceptions, processes them, and then re-raises them with additional information to understand the exception.

Example:

Output:

Explanation: The sys.exc_info() function captures the exception details, allowing us to recognize the error before re-raising it. This helps maintain the original traceback error.

Best Practices for Raising Exceptions in Python

For the proper execution of the Python program, exceptions for error handling have to be raised. In this manner, different exceptions can be raised with the right error display to ensure successful execution. Some of these include the best practices used in Python for raising exceptions, which would improve the readability and efficiency of your programs.

1. Raise Specific Exceptions: For ease of identifying errors, use more specific keywords such as ValueError, TypeError, or KeyError instead of the lesser generic exception.
2. Use Clear Error Messages: The defined error may have to be quite clear on what went wrong during execution.
3. Prefer Built-in Exceptions: Check for a built-in exception before going on to create one, as it helps make your Python code easier.
4. Do Not Raise AssertionError: The use of AssertionError is debugging; it is not well intended for error handling by raising exceptions.
5. Raise Exceptions as Soon as Possible: Raising exceptions directly for the errors instead of checking the code line by line to find the source of the error saves you time.
6. Documentation for Raised Exceptions: If any exception is raised by a certain function within your documentation, it makes it possible for the function to know the type of error expected to have occurred for the exception.

Now let's follow these best practices in the below code:

Example:

Output:

Explanation: Here, the exception is raised if the course name is a valid string from a predefined list and raises an exception (TypeError or ValueError) if it's invalid.

Exceptions vs Syntax Errors in Python

Conclusion

You can raise exceptions in Python using raise, built-in exceptions, specific error messages, or custom exceptions. The assert statement helps in debugging, and the sys.exc_info() allows capturing and re-raising exceptions. Using the right method improves error handling and makes debugging easier. Understanding all these approaches will help you effectively raise an exception in Python.

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