[Python]round四舍五入精度缺失的解决

对python四舍五入的解决方案

现象：一般的四舍五入操作都是使用内置的round方法

In [14]: round(2.675,2)  
Out[14]: 2.67  

文档中这样解释的

The documentation for the built-in round() function says that it rounds to the nearest value, rounding ties away from zero. Since the decimal fraction 2.675 is exactly halfway between 2.67 and 2.68, you might expect the result here to be (a binary approximation to) 2.68. It’s not, because when the decimal string 2.675 is converted to a binary floating-point number, it’s again replaced with a binary approximation, whose exact value is

In [22]: Decimal(2.675)
Out[22]: Decimal('2.67499999999999982236431605997495353221893310546875')

所以对于精度有明确要求的数学计算来说，使用round是不行的

google中有人这么解决的：

>>> from decimal import Decimal
>>> n = Decimal('1.555')
>>> round(n, 2)
Decimal('1.56')

现在使用的方式是：
可以使用str.format来格式化数字实现四舍五入

from decimal import Decimal
In [15]: '{:.2f}'.format(Decimal('2.675'))
Out[15]: '2.68''

错误的写法

In [12]: '{:.2f}'.format(2.675)
Out[12]: '2.67'

In [13]: '{:.2f}'.format('2.675')
-------------------------------------------------------------
ValueError                                Traceback (most recent call last)

In [14]: '{:.2f}'.format(Decimal(2.675))
Out[14]: '2.67'

转自:orangleliu的博客