Errors and exceptions

When something goes wrong, Python raises an exception. You can catch it:

try:
    n = int("forty")
except ValueError:
    n = 0

Common exceptions: ValueError (bad value), ZeroDivisionError (division by zero), KeyError (missing key), IndexError (missing index).

Raise your own exception with raise:

def set_speed(v):
    if v < 0:
        raise ValueError("Speed cannot be negative")
    return v