为什么Python getcontext().prec = 4有时会给出3个小数而不是4个小数? [英] Why Python getcontext().prec = 4 sometimes gives 3 decimals instead of 4?

问题描述

我是python的新手，在我的新旅程中，我使用 decimal 模块遇到了这一点:

I am new to python, and in my new trip I have encountered this using the decimal module:

>>> getcontext().prec = 4; print(Decimal(7)/Decimal(9));
0.7778 # everything ok
>>>
>>> getcontext().prec = 4; print(Decimal(2).sqrt());
1.414 # why 3 and not 4?
>>>
>>> getcontext().prec = 10;    
>>> print(Decimal(10).log10()/Decimal(2).log10()); 
3.321928094 # why 9 if precision is set to 10?

从 https://docs.python.org/2/library/decimal看.html 我没有找到提及的地方.

Looking from the https://docs.python.org/2/library/decimal.html I didn't find a mention on that.

为什么会发生?

感谢您的关注！

推荐答案

猜测:这是有效位数:第二个和第三个示例的小数点前还有一个数字(第一个例子并不重要).

At a guess: it's the number of significant digits: there is also a digit in front of the decimal point for the second and third example (the 0 in the first example is not significant).

请注意，文档中的第四个项目符号为:

Note that the fourth bullet in the documentation says:

十进制模块包含有效位的概念，因此1.30 + 1.20是2.50.

The decimal module incorporates a notion of significant places so that 1.30 + 1.20 is 2.50.

比较(使用第一个示例，但数量更多):

Compare (using your first example, but with a larger number):

>>> getcontext().prec = 8
>>> print Decimal(7000)/Decimal(9)
777.77778
>>> getcontext().prec = 4
>>> print Decimal(7000)/Decimal(9)
777.8
>>> getcontext().prec = 2
>>> print Decimal(7000)/Decimal(9)
7.8E+2

不幸的是，文档中的大多数示例都限于1级数字，因此并不能清楚地显示出来.

Unfortunately, most examples in the documentation are restricted to numbers of order 1, so this doesn't show clearly.

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