If you're in PyCharm and you're getting "Unresolved reference xrange" it's because xrange was removed in Python 3. Range takes over its functionality now. Was working on fast string concat from here:
https://waymoot.org/home/python_string/
And credit for the answer is from /r/learnpython:
https://www.reddit.com/r/learnpython/comments/2udj3s/pycharm_issue/