round Decimal edge case

[ahrvoje]

BPO	39335
Nosy	@benjaminp

^{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 = None
closed_at = <Date 2020-01-15.03:12:42.516>
created_at = <Date 2020-01-14.21:08:15.098>
labels = ['invalid', 'library']
title = 'round Decimal edge case'
updated_at = <Date 2020-01-15.03:12:42.510>
user = 'https://bugs.python.org/ahrvoje'

bugs.python.org fields:

activity = <Date 2020-01-15.03:12:42.510>
actor = 'benjamin.peterson'
assignee = 'none'
closed = True
closed_date = <Date 2020-01-15.03:12:42.516>
closer = 'benjamin.peterson'
components = ['Library (Lib)']
creation = <Date 2020-01-14.21:08:15.098>
creator = 'ahrvoje'
dependencies = []
files = []
hgrepos = []
issue_num = 39335
keywords = []
message_count = 2.0
messages = ['359999', '360021']
nosy_count = 2.0
nosy_names = ['benjamin.peterson', 'ahrvoje']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue39335'
versions = ['Python 2.7']

[ahrvoje]

>>> from decimal import Decimal
>>> round(Decimal('-123.499999999999999999999999999999999999999999'))
-124.0

I would expect -123.0, even considering Py2 rounding convention details (away from zero), Decimal rounding convention (default rounding=ROUND_HALF_EVEN), floating point specifics...

Works as expected in Py3. Both Py2 and Py3 use same default Decimal rounding=ROUND_HALF_EVEN.

Could be I'm missing some detail...

[benjaminp]

On Python 2, round just converts the decimal to a float. So, this is a consequence of:

>>> float(Decimal('-123.499999999999999999999999999999999999999999'))
-123.5