CODEWITHSUNDEEP
pythonpython-2.7floating-point
aifarfa
aifarfa

Reputation: 3939

Best practice to use Python 2.7 decimal in predictable ways (for money)

Firstly, I'm a bit new to Python, I know this floating point arithmetic seems very basic but I can't find any duplicate/related question in SO

I have an acceptance test: expect 3.3 / 3 to be 1.1

Then I tried..

from decimal import *
>>> Decimal(3.3) / Decimal(3)
Decimal('1.099999999999999940788105353')

>>> Decimal(3.3) / Decimal(3.0)
Decimal('1.099999999999999940788105353')

>>> Decimal('3.3') / Decimal('3')
Decimal('1.1') # as expected

Question: What is the best practice to use Python decimal in predictable ways? or that I just need to format every decimal display to string?

To be more specific: I'm writing a small automation script for loan data report.

Upvotes: 1

Views: 165

Answers (2)

Daniel Roseman
Daniel Roseman

Reputation: 599856

The point is that in the bare float 3.3 to Decimal you're already subject to floating-point imprecision:

>>> Decimal(3.3)
Decimal('3.29999999999999982236431605997495353221893310546875')

So, yes, you should always pass strings.

Upvotes: 5

Mirec Miskuf
Mirec Miskuf

Reputation: 1695

Looking at https://docs.python.org/2/library/decimal.html, it is possible to set the precision for your operations. The default is 28 decimal points.

from decimal import *
getcontext().prec = 2
Decimal(3.3) / Decimal(3)

This returns "Decimal('1.1')"

Upvotes: 3

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