from functools import reduce

res = reduce(lambda x, y: x + y, [1, 2, 3, 4, 5])
assert(res == 1 + 2 + 3 + 4 + 5)

res = reduce(lambda x, y: x + y, [1, 2, 3, 4, 5], 10)
assert(res == 10 + 1 + 2 + 3 + 4 + 5)