A pure-Python implementation of Python 2.7's xrange built-in, with some
features backported from the Python 3.x range built-in (which replaced
xrange) in that version.
There is no good reason you should use this in your code, its purpose was to
document and describe the behavior of the xrange or range built-ins, as
well as that of objects implementing the Sequence protocol in general.
Read more at https://late.am/post/2012/06/18/what-the-heck-is-an-xrange