Nitpicking, but this statement doesn't work with large numbers (which xrange() is supposed to handle correctly):

    self._len = int(ceil(float(stop - start) / step))

Python supports arbitrary-length integers; you can't just cast those to (fixed-length) floating point numbers without losing precision. It's better to use integer division here, for example:

    self._len = (stop - start)//step + bool((stop - start)%step)

(A variant like (stop - start + step - 1)//step works only for positive numbers; I guess it could work if you put it in the earlier if-clause and put a corresponding assignment with +1 at the end in the other branch.)