丸めるのと切り下げるの

math_.py

import decimal

def round_n(value, n):
    if n <= 0:
        return decimal.Decimal(str(value)).quantize(decimal.Decimal('1.'), rounding=decimal.ROUND_HALF_UP)
    return decimal.Decimal(str(value)).quantize(decimal.Decimal('.{}1'.format('0' * (n - 1))), rounding=decimal.ROUND_HALF_UP)

def floor_n(value, n):
    if n <= 0:
        return decimal.Decimal(str(value)).quantize(decimal.Decimal('1.'), rounding=decimal.ROUND_FLOOR)
    return decimal.Decimal(str(value)).quantize(decimal.Decimal('.{}1'.format('0' * (n - 1))), rounding=decimal.ROUND_FLOOR)