Python List selecting from back

Question

Suppose I have a list

a = [1,2,3,4,5,6,7]

I want alternate digits starting from second to last ie [6,4,2] .

I tried a[:-1:-2] but I dont get the correct output.

Answer 1

>>> a = [1,2,3,4,5,6,7]
>>> a[-2::-2]
[6, 4, 2]

Answer 2

You need to specify the slice in the format list[start:end:step] so use the following instead:

a[-2::-2] # start at the second to last, to to the end by backing up 2

You are in fact going from at the beginning to the end stepping back by twos, thus getting nothing.