对python四舍五入的处理方案
征象：平常的四舍五入操纵都是运用内置的round要领
In [14]: round(2.675,2)
Out[14]: 2.67
文档中如许诠释的
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')
所以关于精度有明确要求的数学盘算来讲，运用round是不可的
google中有人这么处理的：
>>> from decimal import Decimal
>>> n = Decimal('1.555')
>>> round(n, 2)
Decimal('1.56')
如今运用的体式格局是：
能够运用str.format来格式化数字完成四舍五入
from decimal import Decimal
In [15]: '{:.2f}'.format(Decimal('2.675'))
Out[15]: '2.68''
毛病的写法
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'
