Adding Decimals to classes with OOP + rounding to significant digits (ATM demo)

[bowlofred]

The round context isn't used here because you've passed in a float initially. That float doesn't represent exactly what you want and is already higher than the target. If you want to be exact, you have to pass in a string and let Decimal do the conversion.

>>> Decimal(.505) #.505 isn't exact
Decimal('0.50500000000000000444089209850062616169452667236328125') #Not "5.05".  Is closer to 5.1
>>> round(Decimal(.505),2)
Decimal('0.51')

# Instead of float, pass in a string. '.505' is exact.
>>> Decimal('.505')
Decimal('0.505')
>>> round(Decimal('.505'),2) # now the context matters
Decimal('0.50')
>>> decimal.getcontext().rounding = decimal.ROUND_HALF_UP
>>> round(Decimal('.505'),2)
Decimal('0.51')