[EMAIL PROTECTED] wrote: > The indices method of slice doesn't seem to work quite how I would > expect when reversing a sequence. > > For example : >>>> s = '01234' >>>> s[::-1] > '43210' >>>> s[slice(None,None,-1) ] > '43210' > > So a slice with a negative step (and nothing else) reverses the > sequence. But what are the > corresponding indices? >>>> slice(None,None,-1).indices(len(s)) > (4, -1, -1) > > That looks O.K. The start is the last item in the sequence, and the > stop is one before the beginning of the sequence. But these indices > don't reverse the string: >>>> s[4:-1:-1] > '' > > Although they give the correct range: >>>> range( 4, -1,-1) > [4, 3, 2, 1, 0] > > It would appear that there is no set of indices that will both reverse > the string and produce the correct range! > > Is this a bug or a feature?
I'd say bug in the .indices() method. The meaning of [4:-1:-1] is unavoidable different than [::-1] since the index -1 points to the last element, not the imaginary element before the first element. Unfortunately, there *is* no concrete (start, stop, step) tuple that will emulate [::-1]. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco -- http://mail.python.org/mailman/listinfo/python-list
