You are allowed to raise exceptions yourself. For that, you use the keyword raise, and follow that with one of the known exceptions (potentially, you could create your own, new exceptions if you like, but you need to have studied Chapters 21¹ and 23² before you are ready for it). You can give the exception arguments of any kind that you like.

You might wonder why you would want to raise your own exceptions. The answer is that when you create a module, when an error occurs (for instance because the main program that uses the module passes arguments to a function that are of an incorrect type), it is bad form to print an error message. It is much better to just raise an exception, and let the main program handle the exception. Here is an example of raising an exception:

def getStringLenMax10( prompt ):
    s = input( prompt )
    if len( s ) > 10:
        raise ValueError( "Length exceeds 10", len( s ) )
    return s

print( getStringLenMax10( "Use 10 characters or less: " ) )

When you run this code, you see that if you enter a string of more than 10 characters, a ValueError exception is raised. It has two arguments, which you can see displayed as a tuple when Python splashes the exception on the screen. You can handle this exception just as you would handle exceptions that Python itself produces.

The raise keyword has a second function: if you are in an except clause, and rather than handle the exception there, you want to pass it on to the “next level” of the program, you can just write the keyword raise, and the exception will be “re-raised.” This can be useful when you want to do a bit of extra handling before the program “crashes” or the exception is handled elsewhere. For instance:

fp = open( "pc_rose.txt ")
try:
    buffer = fp.read()
    print( buffer )
except IOError:
    fp.close()
    raise
fp.close()

This code probably runs fine, but if an IOError occurs when reading the file, the file gets closed before the exception is re-raised.