I'd like to overcome an issue with Decimal that i am encountering while migrating from python 2 to 3.
Python 2.7:
>>> Decimal('333').quantize(Decimal('.01'), rounding=decimal.ROUND_HALF_DOWN)
Decimal('333.00')
Python 3.7:
>>> Decimal('333').quantize(Decimal('.01'), rounding=decimal.ROUND_HALF_DOWN)
decimal.InvalidOperation: [<class 'decimal.InvalidOperation'>]
As the exemple show, there is a behavior change between those python version.
Did i miss something there ?
Any insight welcomed.
this append when the decimal.get_context().prec is lower than your expected round
Exemple :
In [3]: from decimal import Decimal
In [4]: Decimal('333').quantize(Decimal('.01'), rounding=decimal.ROUND_HALF_DOWN)
Out[4]: Decimal('333.00')
In [5]: decimal.getcontext()
Out[5]: Context(prec=28, rounding=ROUND_HALF_EVEN, Emin=-999999, Emax=999999, capitals=1, clamp=0, flags=[], traps=[InvalidOperation, DivisionByZero, Overflow])
In [6]: round(Decimal('333'), 2)
Out[6]: Decimal('333.00')
But if you do :
import decimal
In [11]: context = decimal.getcontext()
In [12]: context.prec = 1
In [13]: decimal.setcontext(context)
In [15]: Decimal('333').quantize(Decimal('.01'), rounding=decimal.ROUND_HALF_DOWN)
---------------------------------------------------------------------------
InvalidOperation Traceback (most recent call last)
<ipython-input-15-24094f7b9eb1> in <module>
----> 1 Decimal('333').quantize(Decimal('.01'), rounding=decimal.ROUND_HALF_DOWN)
InvalidOperation: [<class 'decimal.InvalidOperation'>]
So keep in mind that your context.precision need to be greater or equal of your rounding needs