[tonyarkles]

My guess is based on Python 2, where range(n) would return a fully inflated list of 0..(n-1). If len(x) is large, you could be allocating giant temporary lists just to iterate through them once. Using xrange() was the Python 2 solution (it would return a generator instead), but, if I recall correctly, Python 3 fixed this s.t. range() returns a generator.