A special else clause

One of the features I've seen only in the Python language is the ability to have else clauses after while and for loops. It's very rarely used, but it's definitely nice to have. In short, you can have an else suite after a for or while loop. If the loop ends normally, because of exhaustion of the iterator (for loop) or because the condition is finally not met (while loop), then the else suite (if present) is executed. In case execution is interrupted by a break statement, the else clause is not executed. Let's take an example of a for loop that iterates over a group of items, looking for one that would match some condition. In case we don't find at least one that satisfies the condition, we want to raise an exception. This means we want to arrest the regular execution of the program and signal that there was an error, or exception, that we cannot deal with. Exceptions will be the subject of Chapter 8, Testing, Profiling, and Dealing with Exceptions, so don't worry if you don't fully understand them now. Just bear in mind that they will alter the regular flow of the code.

Let me now show you two examples that do exactly the same thing, but one of them is using the special for...else syntax. Say that we want to find, among a collection of people, one that could drive a car:

# for.no.else.py
class DriverException(Exception):
    pass

people = [('James', 17), ('Kirk', 9), ('Lars', 13), ('Robert', 8)]
driver = None
for person, age in people:
    if age >= 18:
        driver = (person, age)
        break

if driver is None:
    raise DriverException('Driver not found.')

Notice the flag pattern again. We set the driver to be None, then if we find one, we update the driver flag, and then, at the end of the loop, we inspect it to see whether one was found. I kind of have the feeling that those kids would drive a very metallic car, but anyway, notice that if a driver is not found, DriverException is raised, signaling to the program that execution cannot continue (we're lacking the driver).

The same functionality can be rewritten a bit more elegantly using the following code:

# for.else.py
class DriverException(Exception):
    pass

people = [('James', 17), ('Kirk', 9), ('Lars', 13), ('Robert', 8)]
for person, age in people:
    if age >= 18:
        driver = (person, age)
        break
else:
    raise DriverException('Driver not found.')

Notice that we aren't forced to use the flag pattern any more. The exception is raised as part of the for loop logic, which makes good sense because the for loop is checking on some condition. All we need is to set up a driver object in case we find one, because the rest of the code is going to use that information somewhere. Notice the code is shorter and more elegant, because the logic is now correctly grouped together where it belongs.