问题
I tried to compute the following by setting getcontext().prec = 800.
>>> from decimal import *
>>> getcontext().prec = 800
>>> Decimal(22.0) / Decimal ( 10.0) - Decimal ( 0.2 )
Decimal('1.999999999999999988897769753748434595763683319091796875')
>>>
But the expected result is 2. Where am I doing wrong?
回答1:
When you construct a Decimal from a floating-point number, you get the exact value of the floating-point number, which may not precisely match the decimal value because that's how floating-point numbers work.
If you want to do precise decimal arithmetic, construct your Decimal objects from strings instead of floating-point numbers:
>>> Decimal('22.0') / Decimal('10.0') - Decimal('0.2')
Decimal('2.0')
回答2:
The prec attribute defines how many numbers after the decimal point will round your number. For example, if you expect 2.00, its value should be 3. Or if you want to round the number so that it has no decimal places you can use 1 as a parameter.
from decimal import *
getcontext().prec = 1
print(Decimal(22.0) / Decimal ( 10.0) - Decimal ( 0.2 ))
>> 2
回答3:
It seems that you have to give numbers as a string to prevent them from being evaluated as floating point.
from decimal import *
getcontext().prec = 800
print(Decimal(0.2))
print(Decimal('0.2'))
print(Decimal('22.0') / Decimal ('10.0') - Decimal ('0.2' ))
which gives
0.200000000000000011102230246251565404236316680908203125
0.2
2.0
回答4:
Pass strings to the Decimal constructor instead of floats: Decimal('0.2') gives the result you expect, Decimal(0.2) doesn't.
This is because:
If value is a float, the binary floating point value is losslessly converted to its exact decimal equivalent. This conversion can often require 53 or more digits of precision. For example, Decimal(float('1.1')) converts to Decimal('1.100000000000000088817841970012523233890533447265625').
https://docs.python.org/3/library/decimal.html#decimal.Decimal
来源：https://stackoverflow.com/questions/62927128/python-decimal-package-gives-wrong-results