Question

Maybe someone can explain why a particular slice operation to reverse
a string doesn’t work. There must be something about slices I don’t
understand. Basic operation: s[start:end:step]

Given

s='abcd'

to reverse this string I can do using default values

s[::-1]

or

s[-1::-1]

Those are fine, but why doesn’t

s[-1:-1:-1]

work and give me the whole string reversed? It should start at with
the last character in the string at (-1), and the stepsize is -1. It should run
down to one before -1 i.e., index 0. Instead I get ”.

for i in xrange(3, -2, -1):
    print 's[-1:%d:-1] => ' %i,
    print s[-1:i:-1]

yields:

s[-1:3:-1] =>     # didn't expect any output
s[-1:2:-1] =>  d
s[-1:1:-1] =>  dc
s[-1:0:-1] =>  dcb
s[-1:-1:-1] =>    # shouldn't this be 'dcba'?

what am I missing here?

(I did see a lot of posting on reversing strings, but if this particular item was explained somewhere I missed it)

Answer 1

s[-1:-1:-1]

Doesn’t work for the same reason that

s[0:0]

doesn’t give you ‘abcd’.

When you slice, you are referring to the space between two elements, not the location of those elements. So you’re asking for the space between -1 and -1, which is ”.