Python - What does this code do?

Question

X = [i for i in range(10)]
r = slice(3, None)
print(r)
print(X[r][r][r])

Answer 1

slice(x, y, z) equals x:y:z, but x:y:z can not be assigned to a variable.

r is just information for lists about how to be sliced.

X[r][r][r]
= X[slice(3, None)][slice(3, None)][slice(3, None)]
= X[3:None][3:None][3:None]

[x:None] equals [x:]

= X[3:][3:][3:]

Then it just gets sliced from left to right:
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9][3:][3:][3:]
= [3, 4, 5, 6, 7, 8, 9][3:][3:]
= [6, 7, 8, 9][3:]
= [9]

Thus X[r][r][r] in the end would equal [9].

Answer 2

Actually you would not need to use nested slices.

You could use calculations to fit multiple slices into 1.

list[a:b:c][d:e:f][g:h:i]
= list[a+b+c : d-a + e-b + f-c : c * f * i]