Round python decimal to nearest 0.05

Question

I'm trying to round money numbers in Decimal to the nearest 0.05. Right now, I'm doing this:

def round_down(amount):
    amount *= 100
    amount = (amount - amount % 5) / Decimal(100)
    return Decimal(amount)

def round_up(amount):
    amount = int(math.ceil(float(100 * amount) / 5)) * 5 / Decimal(100)
    return Decimal(amount)

Is there any way I can do this more elegantly without dealing with floats using python Decimals (using quantize perhaps)?

Answer 1

With floats, simply use round(x * 2, 1) / 2. This doesn't give control over the rounding direction, though.

Using Decimal.quantize you also get complete control over the type and direction of rounding (Python 3.5.1):

>>> from decimal import Decimal, ROUND_UP

>>> x = Decimal("3.426")
>>> (x * 2).quantize(Decimal('.1'), rounding=ROUND_UP) / 2
Decimal('3.45')

>>> x = Decimal("3.456")
>>> (x * 2).quantize(Decimal('.1'), rounding=ROUND_UP) / 2
Decimal('3.5')