Question

Can someone explain why a[:5:-1] != a[:5][::-1]?

>>> a = range(10)
>>> a[:5][::-1]
[4, 3, 2, 1, 0]
>>> a[:5:-1]
[9, 8, 7, 6]

Answer 1

The general syntax of slicings is

a[start:stop:step]

You can omit any of the three values start, stop, or step. If you omit step, it always defaults to 1. The default values of start and stop, by contrast, depend on the sign of step: if step is positive, start defaults to 0 and stop to len(a). If step is negative, start defaults to len(a) - 1 and stop to “beginning of the list”.

So a[:5:-1] is the same as a[9:5:-1] here,

while a[:5][::-1] is the same as a[0:5][4::-1].

(Note that it’s impossible to give the default value for stop explicitly if step is negative. The stop value is non-inclusive, so 0 would be different from “beginning of the list”. Using None would be equivalent to giving no value at all.)