Decimal, quantize, round and negative value

[HakimTaklanti]

BPO	17072
Nosy	@mdickinson

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/mdickinson'
closed_at = <Date 2013-01-29.16:18:38.461>
created_at = <Date 2013-01-29.16:14:16.913>
labels = ['invalid', 'library']
title = 'Decimal, quantize, round and negative value'
updated_at = <Date 2013-02-10.20:40:05.806>
user = 'https://bugs.python.org/HakimTaklanti'

bugs.python.org fields:

activity = <Date 2013-02-10.20:40:05.806>
actor = 'Hakim.Taklanti'
assignee = 'mark.dickinson'
closed = True
closed_date = <Date 2013-01-29.16:18:38.461>
closer = 'mark.dickinson'
components = ['Library (Lib)']
creation = <Date 2013-01-29.16:14:16.913>
creator = 'Hakim.Taklanti'
dependencies = []
files = []
hgrepos = []
issue_num = 17072
keywords = []
message_count = 6.0
messages = ['180911', '180912', '180913', '180914', '181829', '181854']
nosy_count = 2.0
nosy_names = ['mark.dickinson', 'Hakim.Taklanti']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue17072'
versions = ['Python 2.7']

[HakimTaklanti]

>>> from decimal import Decimal
>>> from decimal import ROUND_UP, ROUND_DOWN
>>> a = Decimal("-3.86")
>>> b = Decimal("5.73")
>>> a_up = a.quantize(Decimal(".1"), ROUND_UP)
>>> a.quantize(Decimal(".1"), ROUND_UP) # -3.8 expected
Decimal('-3.9') 
>>> a.quantize(Decimal(".1"), ROUND_DOWN) # -3.9 expected
Decimal('-3.8') 
>>> b.quantize(Decimal(".1"), ROUND_UP) # Ok
Decimal('5.8')
>>> b.quantize(Decimal(".1"), ROUND_DOWN) # Ok
Decimal('5.7')

[mdickinson]

Indeed, that looks wrong. I'll take a look.

[mdickinson]

Sorry, I take that back. The behaviour is correct: ROUND_UP rounds away from zero; ROUND_DOWN towards zero. For rounding towards +/- infinity, you want ROUND_CEILING and ROUND_FLOOR:

Python 2.7.3 |EPD 7.3-1 (32-bit)| (default, Apr 12 2012, 11:28:34) 
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "credits", "demo" or "enthought" for more information.
>>> from decimal import *
>>> a = Decimal("-3.86")
>>> a.quantize(Decimal(".1"), ROUND_CEILING)
Decimal('-3.8')
>>> a.quantize(Decimal(".1"), ROUND_FLOOR)
Decimal('-3.9')

Closing as invalid.

[HakimTaklanti]

Indeed, perhaps to enhance the documentation. Thanks.

[mdickinson]

It's not obvious to me how the documentation could be made clearer: all the rounding modes are described at

http://docs.python.org/2/library/decimal.html#decimal.Context

for the Python 2 docs, and in a separate section entitled 'Rounding modes' for the Python 3 docs:

http://docs.python.org/3/library/decimal.html#rounding-modes

Did you have a particular documentation enhancement in mind?

[HakimTaklanti]

You is right. I had just see the beginning of documentation ("Rounding options include..."). Sorry for the bother and thanks for the answer.