Another exciting corner case in plus/minus:

"The operations are evaluated using the same rules as add and subtract;
 the operations plus(a) and minus(a) (where a and b refer to any numbers)
 are calculated as the operations add(’0’, a) and subtract(’0’, b)
 respectively, where the ’0’ has the same exponent as the operand."


But add and subtract have a special rule for the sign with ROUND_FLOOR:

"Otherwise, the sign of a zero result is 0 unless either both operands 
 were negative or the signs of the operands were different and the 
 rounding is round-floor."


So, +Decimal("-0") and -Decimal("0") should be a negative zero. I checked
this against decNumber. Currently:


>>> c = getcontext()
>>> c.rounding = ROUND_FLOOR
>>> +Decimal("-0")
Decimal('0')
>>> -Decimal("0")
Decimal('0')

Nice catch!  Please could you test the attached patch (against py3k)?

I'll apply it after 3.2 is out.  The fix should also go into the 2.7 maintenance branch, I think.

The patch and the test cases look good, but the implicit context must
be looked up earlier now (see the altered patch).

I just realize that there aren't any regression tests for this sort
of situation, since for the main tests we are only testing the context
methods.

Ah, good point about the context;  thanks for the updated patch.

Yes, those missing regression tests are a problem;  this isn't the first time that the above problem has appeared.

New changeset d5fe8b33f711 by Mark Dickinson in branch '2.7':
Issue 11131: Fix sign of zero result on plus and minus operations in ROUND_FLOOR rounding mode.
http://hg.python.org/cpython/rev/d5fe8b33f711

New changeset 1c4be28cda5b by Mark Dickinson in branch '3.1':
Issue 11131: Fix sign of zero result on decimal.Decimal plus and minus operations in ROUND_FLOOR rounding mode.
http://hg.python.org/cpython/rev/1c4be28cda5b

New changeset 426057570bee by Mark Dickinson in branch '3.2':
Merge #11131
http://hg.python.org/cpython/rev/426057570bee

New changeset af0dcd9df0c0 by Mark Dickinson in branch 'default':
Merge #11131
http://hg.python.org/cpython/rev/af0dcd9df0c0

Fixed in 2.7, 3.1, 3.2, 3.3.  Closing.