Python negative step slice, how, without None, to expresse the equivalent to [::-1]-CodePudding

I've read with interest Understanding slicing and What does 'result[::-1]' mean?

>>> a = 'Python'
# a[start:stop:step]  with start and stop inverted if stem < 1
>>> a[::-1]
'nohtyP'

I got that:

>>> a[::-1] == a[None:None:-1]
True

But I still asking to myself the following question:

Why I cant create with some default start stop values the equivalent of a[::-1] ?

>>> a[len(a):0:-1]
'nohty'
>>> a[len(a) 1:0:-1]
'nohty'
>>> a[len(a) 1:-1:-1]
''

Can I catch the P of Python with explicit indices ?

CodePudding user response：

You can reverse the string with explicit indices if you use negative indices

print(a[-1:-len(a)-1:-1]) # nohtyP

CodePudding user response：

I think not because the stop value in the slice is not included in the slice.

You are looking for a stop value that includes P, in other words includes index 0. However that is not possible due to how python uses negative numbers in slices.

a[-1:0:-1] -> 'nohty'

We would expect to get P if we go down by one from 0, use -1, but -1 is interpreted as last index, hence the string will be empty.

a[-1:-1:-1] -> ''

TL;DR: nope, not possible