Occasionally it happens that you want to exit a program early when a certain condition arises. For instance, your program asks the user for a value, and then processes that value extensively. But if the user enters a value that cannot be processed, the program should just end. You could code that as follows:

from pcinput import getInteger

num = getInteger( "Please enter a positive integer: " )
if num < 0:
    print( "You should have entered a positive integer!" )
else:
    print( "Now I am processing your integer", num )
    print( "Lots and lots of processing" )
    print( "Hundreds of lines of code here" )

It is a bit irritating that most of your program is already one indent deep, while you would have preferred to leave the program at the error message, and then have the rest of the program at the top indent level.

You can do that using a special function exit() that is found in the module sys. The code above becomes:

from pcinput import getInteger
from sys import exit

num = getInteger( "Please enter a positive integer: " )
if num < 0:
    print( "You should have entered a positive integer!" )
    exit()

print( "Now I am processing your integer", num )
print( "Lots and lots of processing" )
print( "Hundreds of lines of code here" )

When you run this code and enter a negative number, depending on which editor you use (IDLE does not have this issue), you may find that Python raises a SystemExit exception, which looks like a big, ugly error. It is not, however. This exception just says that you forced the program to end, but that is exactly what you wanted. This is actually a nice, clean exit.

In general, you are not allowed to ignore error messages and warnings. This one is the exception to the rule. You are allowed to exit your program this way. In Chapter 9¹ I will explain how you can suppress this ugly message (if you get it and if you really want to), but for now, just accept it.