round_amounts.py

# https://stackoverflow.com/questions/15769948/round-a-python-list-of-numbers-and-maintain-the-sum

from decimal import Decimal, ROUND_HALF_UP


def round_amounts(amounts, original_amount, decimal_places):
    amounts = [Decimal(str(amount)) for amount in amounts]
    places = Decimal(10) ** -decimal_places
    rounded_list = (
        [amount.quantize(places, ROUND_HALF_UP) for amount in amounts])
    rounded_list_sum = sum(rounded_list)
    error = Decimal(original_amount) - rounded_list_sum
    n = int(round(error / places))
    new_list = rounded_list[:]
    for _, i in sorted(
        ((amounts[i] - rounded_list[i], i)
         for i in range(len(amounts))), reverse=n > 0)[:abs(n)]:
        new_list[i] += Decimal.copy_sign(places, n)

    _check_amounts_sum(new_list, original_amount)
    return new_list


def round_amounts_by_ratio(ratios, original_amount, decimal_places):
    amounts = [(Decimal(original_amount * ratio).
                quantize(decimal_places, ROUND_HALF_UP))
               for ratio in ratios]
    amounts_rounded = round_amounts(
        amounts=amounts, original_amount=original_amount,
        decimal_places=decimal_places)
    return amounts_rounded


def _check_amounts_sum(amounts, original_amount):
    amounts_sum = sum(amounts)
    error = amounts_sum - original_amount
    if abs(error) > 0:
        message = ('Sum of amounts ({0}) != original ({1})'.
                   format(amounts_sum, original_amount))
        raise ValueError(message)