Repeat list if index range is out of bounds

Question

I have a Python list

a = [1, 2, 3, 4]

and I'd like to get a range of indices such that if I select the indices 0 through N, I'm getting (for N=10) the repeated

[1, 2, 3, 4, 1, 2, 3, 4, 1, 2]

I could of course repeat the list via (int(float(N) / len(a) - 0.5) + 1) * a first and select the range [0:10] out of that, but that feels rather clumsy.

Any hints?

Answer 1

You can simply use the modulo operator when accessing the list, i.e.

a[i % len(a)]

This will give you the same result, but doesn't require to actually store the redundant elements.

Answer 2

You can use itertools.cycle and itertools.islice:

from itertools import cycle, islice

my_list = list(islice(cycle(my_list), 10))

Note that if you just want to iterate over this once, you should avoid calling list and just iterate over the iterable, since this avoids allocating repeated elements.