python实现小数精确四舍五入

一、现象描述

由于 python3 包括python2.7 以后的round策略使用的是decimal.ROUND_HALF_EVEN
即Round to nearest with ties going to nearest even integer. 也就是只有在整数部分是奇数的时候， 小数部分才逢5进1； 偶数时逢5舍去。 这有利于更好地保证数据的精确性， 并在实验数据处理中广为使用。

二、修改decimal上线文，实现一般意义上的精确四舍五入

ROUND_HALF_UP (to nearest with ties going away from zero), or

import decimal
from decimal import Decimal

print(decimal.getcontext())

context=decimal.getcontext() # 获取decimal现在的上下文
context.rounding = decimal.ROUND_HALF_UP #修改rounding策略

print(decimal.getcontext())

print(round(Decimal(1.5659898), 2))
print(round(Decimal(1.5344898), 2))
print(round(Decimal(2.5659898), 2))
print(round(Decimal(2.5359898), 2))

# with decimal.localcontext() as ctx:
#     ctx.prec = 32
#     ctx.rounding=decimal.ROUND_HALF_UP
#     print(round(decimal.Decimal(2.669), 2))
#     print(round(decimal.Decimal(1.635), 2))

运行结果：

Context(prec=28, rounding=ROUND_HALF_EVEN, Emin=-999999, Emax=999999, capitals=1, clamp=0, flags=[], traps=[InvalidOperation, DivisionByZero, Overflow])
1.57
1.53
2.57
2.54
Context(prec=28, rounding=ROUND_HALF_UP, Emin=-999999, Emax=999999, capitals=1, clamp=0, flags=[Inexact, FloatOperation, Rounded], traps=[InvalidOperation, DivisionByZero, Overflow])

三、自定义四舍五入方法实现精确四舍五入

def round_up(number,power=0):
    """
    实现精确四舍五入，包含正、负小数多种场景
    :param number: 需要四舍五入的小数
    :param power: 四舍五入位数，支持0-∞
    :return: 返回四舍五入后的结果
    """
    digit = 10 ** power
    num2 = float(int(number * digit))
    # 处理正数，power不为0的情况
    if number>=0 and power !=0:
        tag = number * digit - num2 + 1 / (digit * 10)
        if tag>=0.5:
            return (num2+1)/digit
        else:
            return num2/digit
    # 处理正数，power为0取整的情况
    elif  number>=0 and power==0 :
        tag = number * digit - int(number)
        if tag >= 0.5:
            return (num2 + 1) / digit
        else:
            return num2 / digit
    # 处理负数，power为0取整的情况
    elif power==0 and number<0:
        tag = number * digit - int(number)
        if tag <= -0.5:
            return (num2 - 1) / digit
        else:
            return num2 / digit
    # 处理负数，power不为0的情况
    else:
        tag = number * digit - num2 - 1 / (digit * 10)
        if tag <= -0.5:
            return (num2-1)/digit
        else:
            return num2/digit


print(round_up( 2.4259, 0))
print(round_up( -2.625, 2))
print(round_up( -2.635, 2))
print(round_up( 0, 0))

结果示例：
2.0
-2.63
-2.64
0.0

参考：

https://docs.python.org/2/library/decimal.html