问为什么下面这种情况下的切片运算符在python中不起作用？

EN

step value can be either +ve or -ve
-----------------------------------------
if +ve then it should be forward direction (L to R)
if -ve then it should be backward direction(R to L)

if +ve forward direction from begin to end-1
if -ve backward direction from begin to end + 1

in forward direction
-------------------------------
default : begin : 0
default : end : length of string
default step : 1

in backward direction
---------------------------------
default begin : -1
default end : -(len(string) + 1)

>>> x = '0123456789'
>>> x[2:-1:-1]
''
>>> x[2:0:-1]
'21'