I want to display:
49 as 49.00
and:
54.9 as 54.90
Regardless of the length of the decimal or whether there are are any decimal places, I would like to display a Decimal with 2 decimal places, and I'd like to do it in an efficient way. The purpose is to display money values.
eg, 4898489.00
For the analogous issue with the built-in float type, see Limiting floats to two decimal points.
I suppose you're probably using the Decimal() objects from the decimal module? (If you need exactly two digits of precision beyond the decimal point with arbitrarily large numbers, you definitely should be, and that's what your question's title suggests...)
If so, the Decimal FAQ section of the docs has a question/answer pair which may be useful for you:
Q. In a fixed-point application with two decimal places, some inputs have many places and need to be rounded. Others are not supposed to have excess digits and need to be validated. What methods should be used?
A. The quantize() method rounds to a fixed number of decimal places. If the Inexact trap is set, it is also useful for validation:
>>> TWOPLACES = Decimal(10) ** -2 # same as Decimal('0.01')
>>> # Round to two places
>>> Decimal('3.214').quantize(TWOPLACES)
Decimal('3.21')
>>> # Validate that a number does not exceed two places
>>> Decimal('3.21').quantize(TWOPLACES, context=Context(traps=[Inexact]))
Decimal('3.21')
>>> Decimal('3.214').quantize(TWOPLACES, context=Context(traps=[Inexact]))
Traceback (most recent call last):
...
Inexact: None
The next question reads
Q. Once I have valid two place inputs, how do I maintain that invariant throughout an application?
If you need the answer to that (along with lots of other useful information), see the aforementioned section of the docs. Also, if you keep your Decimals with two digits of precision beyond the decimal point (meaning as much precision as is necessary to keep all digits to the left of the decimal point and two to the right of it and no more...), then converting them to strings with str will work fine:
str(Decimal('10'))
# -> '10'
str(Decimal('10.00'))
# -> '10.00'
str(Decimal('10.000'))
# -> '10.000'