Python decimal rounding behavior

gistfile1.txt

-2.695
    reduced by ROUND_UP, ROUND_FLOOR, ROUND_HALF_UP, ROUND_HALF_EVEN
    increased by ROUND_DOWN, ROUND_05UP, ROUND_CEILING, ROUND_HALF_DOWN
-2.669, -2.699
    reduced by ROUND_UP, ROUND_FLOOR, ROUND_HALF_UP, ROUND_HALF_DOWN, ROUND_HALF_EVEN
    increased by ROUND_DOWN, ROUND_05UP, ROUND_CEILING
-2.665
    reduced by ROUND_UP, ROUND_FLOOR, ROUND_HALF_UP
    increased by ROUND_DOWN, ROUND_05UP, ROUND_CEILING, ROUND_HALF_DOWN, ROUND_HALF_EVEN
-2.661, -2.691
    reduced by ROUND_UP, ROUND_FLOOR
    increased by ROUND_DOWN, ROUND_05UP, ROUND_CEILING, ROUND_HALF_UP, ROUND_HALF_DOWN, ROUND_HALF_EVEN
-2.655
    reduced by ROUND_UP, ROUND_05UP, ROUND_FLOOR, ROUND_HALF_UP, ROUND_HALF_EVEN
    increased by ROUND_DOWN, ROUND_CEILING, ROUND_HALF_DOWN
-2.609, -2.659
    reduced by ROUND_UP, ROUND_05UP, ROUND_FLOOR, ROUND_HALF_UP, ROUND_HALF_DOWN, ROUND_HALF_EVEN
    increased by ROUND_DOWN, ROUND_CEILING
-2.605
    reduced by ROUND_UP, ROUND_05UP, ROUND_FLOOR, ROUND_HALF_UP
    increased by ROUND_DOWN, ROUND_CEILING, ROUND_HALF_DOWN, ROUND_HALF_EVEN
-2.601, -2.651
    reduced by ROUND_UP, ROUND_05UP, ROUND_FLOOR
    increased by ROUND_DOWN, ROUND_CEILING, ROUND_HALF_UP, ROUND_HALF_DOWN, ROUND_HALF_EVEN
2.601, 2.651
    reduced by ROUND_DOWN, ROUND_FLOOR, ROUND_HALF_UP, ROUND_HALF_DOWN, ROUND_HALF_EVEN
    increased by ROUND_UP, ROUND_05UP, ROUND_CEILING
2.605
    reduced by ROUND_DOWN, ROUND_FLOOR, ROUND_HALF_DOWN, ROUND_HALF_EVEN
    increased by ROUND_UP, ROUND_05UP, ROUND_CEILING, ROUND_HALF_UP
2.609, 2.659
    reduced by ROUND_DOWN, ROUND_FLOOR
    increased by ROUND_UP, ROUND_05UP, ROUND_CEILING, ROUND_HALF_UP, ROUND_HALF_DOWN, ROUND_HALF_EVEN
2.655
    reduced by ROUND_DOWN, ROUND_FLOOR, ROUND_HALF_DOWN
    increased by ROUND_UP, ROUND_05UP, ROUND_CEILING, ROUND_HALF_UP, ROUND_HALF_EVEN
2.661, 2.691
    reduced by ROUND_DOWN, ROUND_05UP, ROUND_FLOOR, ROUND_HALF_UP, ROUND_HALF_DOWN, ROUND_HALF_EVEN
    increased by ROUND_UP, ROUND_CEILING
2.665
    reduced by ROUND_DOWN, ROUND_05UP, ROUND_FLOOR, ROUND_HALF_DOWN, ROUND_HALF_EVEN
    increased by ROUND_UP, ROUND_CEILING, ROUND_HALF_UP
2.669, 2.699
    reduced by ROUND_DOWN, ROUND_05UP, ROUND_FLOOR
    increased by ROUND_UP, ROUND_CEILING, ROUND_HALF_UP, ROUND_HALF_DOWN, ROUND_HALF_EVEN
2.695
    reduced by ROUND_DOWN, ROUND_05UP, ROUND_FLOOR, ROUND_HALF_DOWN
    increased by ROUND_UP, ROUND_CEILING, ROUND_HALF_UP, ROUND_HALF_EVEN

test_quantize.py

import decimal as d
from collections import defaultdict
from operator import itemgetter

exp = d.Decimal("0.01")

numbers = map(d.Decimal, [
    "-2.601", "-2.605", "-2.609",
    "-2.651", "-2.655", "-2.659",
    "-2.661", "-2.665", "-2.669",
    "-2.691", "-2.695", "-2.699",
    
    "2.601", "2.605", "2.609",
    "2.651", "2.655", "2.659",
    "2.661", "2.665", "2.669",
    "2.691", "2.695", "2.699",
])

roundings = [
    d.ROUND_UP,
    d.ROUND_DOWN,
    d.ROUND_05UP,
    d.ROUND_FLOOR,      
    d.ROUND_CEILING,
    d.ROUND_HALF_UP,
    d.ROUND_HALF_DOWN,
    d.ROUND_HALF_EVEN,  
]

groups = defaultdict(list)

for n in numbers:
    reduced, increased = [], []
    for r in roundings:
        q = n.quantize(exp, r)
        if q > n:
            increased.append(r)
        elif q < n:
            reduced.append(r)
    if reduced or increased:
        groups[tuple(reduced), tuple(increased)].append(n)

assert len(groups) <= 16, len(groups)

for (reduced, increased), group in sorted(groups.iteritems(), key=itemgetter(1)):
    print ", ".join(map(str, group))
    print "    reduced by", ", ".join(reduced)
    print "    increased by", ", ".join(increased)

That was really helpful, thanks!