In short, it's the default behavior in Python 3; it doesn't exist anymore:

http://pythoncentral.io/how-to-use-pythons-xrange-and-range/

From that article (buried near the end for some reason):

>One more thing to add. In Python 3.x, the xrange function does not exist anymore. The range function now does what xrange does in Python 2.x, so to keep your code portable, you might want to stick to using range instead.