Unfortunatly you cannot do this with slicing, you'll need to concatonate to two segments:
import numpy as np
a = [1, 2, 3, 4, 5, 6]
if starting_index > ending_index:
part1 = a[start_index:]
part2 = a[:end_index]
result = np.concatenate([part1, part2])
else:
result = a[start_index:end_index]