[-1:0:-1] means: start from the index len(string)-1 and move up to 0(not included) and take a step of -1(reverse).
So, the following indexes are fetched:
le-1, le-1-1, le-1-1-1 .... 1 # le is len(string)
example:
In [24]: strs = 'foobar'
In [25]: le = len(strs)
In [26]: strs[-1:0:-1] # the first -1 is equivalent to len(strs)-1
Out[26]: 'raboo'
In [27]: strs[le-1:0:-1]
Out[27]: 'raboo'
