string - Python reverse-stride slicing

Question

A specific example of my question is, "How can I get '3210' in this example?"

>>> foo = '0123456'
>>> foo[0:4]
'0123'
>>> foo[::-1]
'6543210'
>>> foo[4:0:-1] # I was shooting for '3210' but made a fencepost error, that's fine, but...
'4321'
>>> foo[3:-1:-1] # How can I get '3210'?
''
>>> foo[3:0:-1]
'321'

It seems strange that I can write foo[4:0:-1], foo[5:1:-1], etc. and get what I would expect, but there's no way to write the slice so that I get '3210'.

A makeshift way of doing this would be foo[0:4][::-1], but this creates two string objects in the process. I will be performing this operation literally billions of times, so every string operation is expensive.

I must be missing something silly and easy. Thanks for your help!

Answer 1

Simply exclude the end range index...

>>> foo[3::-1]
'3210'

Ironically, about the only option I think you didn't try.