反向切片Python

Question

In Python I know you can slice sequences as seq[from:to+1:step]. But if you slice in reverse, using a step of -1, why does it not show the from element? It shifts one element to the right from what I would expect.

x="012345"
x[4:2:-1] 

prints 43, rather than 4321 as I expected. So if step is -1 then it prints seq[from:to-1:step]. Seems inconsistent.

Answer 1

If you want to slice down to 4321 you should do x[-2:0:-1] .

This means to start from -2 (-1 is the last element) and go to 0 (nothing is the first element) with -1 as step.

Answer 2

It works as expected.
If you need your result,

>>> x[4:0:-1]
'4321'

This means, start slicing from 5th element and go till 1st element(but not including it). This kind of slicing works only if you specify the step size to be negative.

So in general, follow olofom's answer.

Answer 3

Slicing goes from the first number (inclusive) to the second number (exclusive) with a step of the third number. In your case, the first number accesses "4" and the second number accesses "2". With a step of -1, it will start with the character at the position of the first number ("4"), and then get the next character (or with a step of -1, actually the previous character "3"), but will stop there because the second index (2) is exclusive. To get everything from "4" to "0", you can do x[4:0:-1] or, more conveniently, x[:5] .