Given a list of integers, determine the index of the least upper bound of some value that is in the cumulative sum array of the list.

find_lub_index_cumsum.py

import bisect
import itertools

def find_lub_cumsum_index(a, x):
    a_cumsum = list(itertools.accumulate(a))

    i = bisect.bisect_right(a_cumsum, x)
    if i != len(a):
            return i
    raise ValueError

if __name__ == '__main__':
    import unittest

    class test_find_lub_cumsum_index(unittest.TestCase):
        @classmethod
        def setUpClass(cls):
            cls.arr = [30, 15, 20]

        def test_within_bounds(self):
            self.assertEqual(find_lub_cumsum_index(self.arr, 20), 0)
            self.assertEqual(find_lub_cumsum_index(self.arr, 40), 1)
            self.assertEqual(find_lub_cumsum_index(self.arr, 45), 2)

        def test_above_bounds(self):
            with self.assertRaises(ValueError):
                find_lub_cumsum_index(self.arr, 65)

    unittest.main()