Why output of this code is 9? Plz explain line 2 & 3.

Question

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

Answer 1

Line 2 is equivalent to writing x[3:].

So it gets all the elements from index 3 to the end.

If you apply the same pattern again on the resulting list, you jump 3 steps further forward, the resulting list will be smaller. And then once again.

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9][3:]
->[3, 4, 5, 6, 7, 8, 9][3:]
->[6, 7, 8, 9][3:]
->[9]

Answer 2

x[slice(3, None)]
equals:
x[3:]

You were trying to make a slice:
x[3:][3:][3:]

And you need to take slice after slice.

x = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

x[3:][3:][3:]
-> [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]