from decimal import getcontext, Inexact, Decimal

def decimalsum(iterable, start=Decimal('0')):
    '''Exact sum of Decimal/int/float mix; Result is *unrounded*'''
    if not isinstance(start, Decimal):
        start = Decimal(start)
    # We need our own context and we can't just set it once because
    # the loop could be over a generator/iterator/coroutine
    ctx = getcontext().copy()
    ctx.traps[Inexact] = True
    one = Decimal(1)

    total = start
    for x in iterable:
        if not isinstance(x, Decimal):
            x = Decimal(x)
        # Increase the precision until we get an exact result.
        while True:
            try:
                total = total.fma(one, x, ctx)
                break
            except Inexact:
                ctx.prec *= 2

    # Result is exact and unrounded.
    return total


D = Decimal
assert decimalsum([D("1.02"), 3e100, D("0.98"), -3e100]) == 2