python实现小数精确四舍五入

以你之姓@ 2022-02-21 04:19 235阅读 0赞

### 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。  
![在这里插入图片描述][20200430154651410.png]

### 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

[20200430154651410.png]: /images/20220221/31f69c88f24e49dfaa8dedb675dd7ec0.png