Python ROUND_HALF_UP无法正常工作 [英] Python ROUND_HALF_UP not working as expected

问题描述

我似乎无法理解十进制文档，使用十进制类将2.775舍入为2.78。

I can't seem to understand the decimal docs to get 2.775 to round to 2.78 using the decimal class.

import decimal
decimal.getcontext().prec = 7
print(decimal.Decimal(2.775).quantize(
    decimal.Decimal('0.00'), 
    rounding=decimal.ROUND_HALF_UP
))
>> 2.77 # I'm expecting 2.78

该值应该取整为2.78，但我一直得到2.77。

That SHOULD round to 2.78, but I keep getting 2.77.

编辑：
在python 3.4上进行测试

Testing on python 3.4

推荐答案

如果在代码中添加一行，这样：

If you add a single line to your code thus:

print (decimal.Decimal(2.775))

然后您会看到为什么向下取整：

then you'll see why it's rounding down:

2.774999999999999911182158029987476766109466552734375

值 2.775 ，因为字面量存储为精度有限的 double 。

The value 2.775, as a literal is stored as a double, which has limited precision.

如果您指定它，作为字符串，该值将更准确地保留：

If you instead specify it as a string, the value will be preserved more accurately:

>>> import decimal

>>> print (decimal.Decimal(2.775))
2.774999999999999911182158029987476766109466552734375

>>> print (decimal.Decimal('2.775'))
2.775

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