Raising your own errors

When a function receives something it cannot handle, raise an exception instead of quietly returning a wrong answer: raise ValueError("message"). The caller decides what to do with it using try/except. For problems specific to your program, create a custom exception by subclassing Exception.

How it reads

• raise stops the function immediately and sends the error up to the caller
• class LaunchError(Exception) makes a new kind of error you can catch by name
• except ... as e gives you the error, and print(e) shows its message

Cloud tip: Raise early, catch late: check inputs at the top of a function, and handle errors where you can actually do something about them.