How to round Python Decimal instance

Question

How do I round a Python Decimal instance to a specific number of digits while rounding to the nearest decimal?

I've tried using the .quantize(Decimal('.01')) method outlined in the docs, and suggested in previous answers, but it doesn't seem to round correctly despite trying different ROUND_ options. I've also tried setting getcontext().prec, but that seems to only control the total number of digits in the entire number, not just the decimals.

e.g. I'm trying to do something like:

assert Decimal('3.605').round(2) == Decimal('3.61')
assert Decimal('29342398479823.605').round(2) == Decimal('29342398479823.61')
assert Decimal('3.604').round(2) == Decimal('3.60')
assert Decimal('3.606').round(2) == Decimal('3.61')

Mark Ransom · Accepted Answer · 2013-04-05 02:12:31Z

8

I think you need to use the decimal.ROUND_HALF_UP option to quantize to get what you want.

>>> for x in ('3.605', '29342398479823.605', '3.604', '3.606'):
    print x, repr(Decimal(x).quantize(Decimal('.01'), decimal.ROUND_HALF_UP))

3.605 Decimal('3.61')
29342398479823.605 Decimal('29342398479823.61')
3.604 Decimal('3.60')
3.606 Decimal('3.61')

answered Apr 5, 2013 at 2:12

Mark Ransom

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