Someone pointed out that this isn't correct, and then deleted their comment. (?)

I now think the behiviour I ascribed to print, is actually what repr does. I'm not sure about print, but I guess it is just rounding to a certain number of places.

    >>> from decimal import Decimal

    >>> Decimal(0.1+0.2)
    Decimal('0.3000000000000000444089209850062616169452667236328125')

    >>> Decimal(0.3)
    Decimal('0.299999999999999988897769753748434595763683319091796875')

    >>> Decimal(0.300000000000000000000000000001)
    Decimal('0.299999999999999988897769753748434595763683319091796875')

    >>> print 0.300000000000000000000000000001
    0.3

    >>> repr(0.300000000000000000000000000001)
    '0.3'

    >>> D(0.30000000000000004)
    Decimal('0.3000000000000000444089209850062616169452667236328125')

    >>> repr(0.299999999999999988897769753748434595763683319091796875)
    '0.3'

    >>> repr(0.30000000000000004)
    '0.30000000000000004'

    >>> print 0.30000000000000004
    0.3

you are using Decimal "wrong". If you want it to handle exact values, you need to pass a string with the decimal interpretation

    Python 3.5.0 (v3.5.0:374f501f4567, Sep 13 2015, 02:27:37) [MSC v.1900 64 bit (AMD64)] on win32
    Type "help", "copyright", "credits" or "license" for more information.
    >>> from decimal import Decimal
    >>> Decimal('0.1') + Decimal('0.2') - Decimal('0.3')
    Decimal('0.0')
    >>> Decimal(0.1) + Decimal(0.2) - Decimal(0.3)
    Decimal('2.775557561565156540423631668E-17')
    >>> Decimal(0.1) == Decimal('0.1')
    False

In this case, that was deliberate. I was using it to show the exact decimal value of the floating point approximations to decimal numbers, not to do decimal arithmetic.

There's some discussion here: https://docs.python.org/3.1/tutorial/floatingpoint.html. Basically: eval(repr(x)) == x is guaranteed, and some attempt is made to give the shortest such string representation for Python 3.1+ (and 2.7?), but it's not guaranteed on all platforms.

It also matters whether you're using Python 2 or 3. This is one of the things they changed in 3:

    >>> print(0.1 + 0.2)
    0.30000000000000004

I believe this is relevant: https://bugs.python.org/issue1580