[issue1979] Make Decimal comparisons with NaN less arbitrary

[Mark Dickinson]

New submission from Mark Dickinson:

For Python 3.0 the decimal module got rich comparisons.  Those comparisons have somewhat unconventional 
behavior with respect to NaNs, as seen below:  <, <= and == comparisons involving NaNs always return False, 
while >, >= and != comparisons always return True.

The Decimal specification has nothing helpful to say about comparisons involving NaNs.  But reading IEEE-
754r (on which the Decimal specification is closely based), there are two possible options:

 (1) have comparisons involving NaNs (except for !=) raise InvalidOperation in the context, and hence give a 
Python exception (assuming that InvalidOperation isn't trapped.)

 (2) have comparisons involving NaNs always return False (except for !=, which always returns True).

I think either of these is better than the current behavior.  (2) seems like the better option, for a couple 
of reasons:  first, it's the way that Python floats currently work, and second, there might be issues with 
list membership testing if equality comparisons involving NaNs raised an exception or returned a NaN.

Since Mike Cowlishaw is intimately involved with both the Decimal specification and the IEEE-754r process, I 
thought it might be useful to have his opinion on this;  his main recommendation was to have the Decimal 
type do the same as the float type.

The attached patch makes <, <=, >, >= and == comparisons involving NaNs always return False, and != 
comparisons always return True.  It also renames __cmp__ to _cmp and adds a few tests for the new behavior.

Here's how NaN comparisons currently work:

Python 3.0a2+ (py3k:60470M, Jan 30 2008, 23:11:40) 
[GCC 4.0.1 (Apple Inc. build 5465)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from decimal import *
>>> n = Decimal("nan")
>>> i = Decimal("inf")
>>> n < n
False
>>> n > n
True
>>> i < n
False
>>> i > n
True

See also issue #1514428.

----------
assignee: facundobatista
components: Library (Lib)
files: richcmp.patch
keywords: patch
messages: 61884
nosy: facundobatista, marketdickinson
severity: minor
status: open
title: Make Decimal comparisons with NaN less arbitrary
type: behavior
versions: Python 3.0
Added file: http://bugs.python.org/file9337/richcmp.patch

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