Python - 关于十进制运算的问题

Question

我有三个关于Python中小数运算的问题，这三个问题最好直接在这里问：

1)

>>> from decimal import getcontext, Decimal
>>> getcontext().prec = 6
>>> Decimal('50.567898491579878') * 1
Decimal('50.5679')
>>> # How is this a precision of 6? If the decimal counts whole numbers as
>>> # part of the precision, is that actually still precision?
>>> 

还有

2)

>>> from decimal import getcontext, Decimal
>>> getcontext().prec = 6
>>> Decimal('50.567898491579878')
Decimal('50.567898491579878')
>>> # Shouldn't that have been rounded to 6 digits on instantiation?
>>> Decimal('50.567898491579878') * 1
Decimal('50.5679')
>>> # Instead, it only follows my precision setting set when operated on.
>>> 

3)

>>> # Now I want to save the value to my database as a "total" with 2 places.
>>> from decimal import Decimal
>>> # Is the following the correct way to get the value into 2 decimal places,
>>> # or is there a "better" way?
>>> x = Decimal('50.5679').quantize(Decimal('0.00'))
>>> x  # Just wanted to see what the value was
Decimal('50.57')
>>> foo_save_value_to_db(x)
>>> 

Answer 1

1. 精度是根据有效数字来决定的，而不是小数位数。有效数字在科学应用中更有用。

2. 原始数据绝不能被随便修改。应该在处理数据时再进行修改。

3. 这就是操作的方法。