python decimal quantize vs prec in context

Question

Consider the following rounding approaches in decimal:

using quantize:

>>> (Decimal('1')/Decimal('3')).quantize(Decimal('0.00'), rounding=ROUND_HALF_UP)
Decimal('0.33')

using context:

>>> ctx = Context(prec=2, rounding=ROUND_HALF_UP)
>>> setcontext(ctx)
>>> Decimal('1')/Decimal('3')
Decimal('0.33')

Are there any actual differences between the 2 methods for rounding? Any gotchas? Is using the context a bit more elegant so that I can use with statement for the whole calculation block?

Answer 1

>>> from decimal import Decimal, ROUND_HALF_UP, setcontext, Context
>>> ctx = Context(prec=2, rounding=ROUND_HALF_UP)
>>> setcontext(ctx)
>>> total = Decimal('0.002') + Decimal('0.002')
>>> total
Decimal('0.004')

It actually does not round automatically as I would have expected, so I won't be able to use that for the whole calculation block.

Another problem, interim values are rounded which loses precision.

from decimal import Decimal, ROUND_HALF_UP, getcontext, setcontext, Context

class FinanceContext:
    def __enter__(self):
        self.old_ctx = getcontext()
        ctx = Context(prec=2, rounding=ROUND_HALF_UP)
        setcontext(ctx)
        return ctx

    def __exit__(self, type, value, traceback):
        setcontext(self.old_ctx)


class Cart(object):
    @property
    def calculation(self):
        with FinanceContext():
            interim_value = Decimal('1') / Decimal('3')
            print interim_value, "prints 0.33, lost precision due to new context"

            # complex calculation using interim_value
            final_result = ...
            return final_result