What does the 'raise' keyword do in Python?

The 'raise' keyword in Python is used to raise an exception. This keyword effectively interrupts the normal flow of a program by triggering a specific exception. When an exception is raised, it can either be a predefined exception type or a custom one defined by the user. This mechanism allows programmers to signal that an error condition has occurred, providing a way to handle errors gracefully through the use of exception handling blocks.

For example, using 'raise' can help in validating conditions within a function. If an argument falls outside an expected range, you might raise a ValueError to inform the calling code of the problem. This not only makes the code easier to debug but also enforces clearer contracts for function behavior.

The choice regarding creating a new function does not accurately describe the functionality of 'raise', as it does not serve to define functions. Calling a built-in exception might be somewhat close but does not capture the specificity of 'raise', which is about the act of raising exceptions rather than merely invoking built-in values. Lastly, handling an existing exception refers to responding to exceptions that have already been raised, often using a try-except block, which is a separate concept from the act of initiating an exception.