[ python-Bugs-1729014 ] 0.0 and -0.0 end up referring to the same object

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Bugs item #1729014, was opened at 2007-05-31 15:27
Message generated for change (Comment added) made by swfiua
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Category: Python Interpreter Core
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Johnnyg (swfiua)
Assigned to: Nobody/Anonymous (nobody)
Summary: 0.0 and -0.0 end up referring to the same object

Initial Comment:
I am not really sure whether this is a bug or a feature.

The attached code attempts to demonstrate the problem.

I had some code that was trying to change -0.0 to 0.0 so that the accountants I work with don't panic.

The code was something like this:

if n == -0.0:
    n = 0.0

Regardless of whether n is -0.0 or 0.0 the test passes (which is good).

However after the assignment n is actually -0.0

It looks like python is creating a single object for both -0.0 and 0.0.

Whichever appears first within the local scope seems to be the value that actually gets stored.

Eg changing the code to 

if n == 0.0:
    n = 0.0

gets me the behaviour I wanted.

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>Comment By: Johnnyg (swfiua)
Date: 2007-06-05 10:52

Message:
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I'm happy to flag this as undefined behaviour.

I have worked around it in my code, the only issue is that the code is
brittle, since I think it relies on the scope of constants -- I'm guessing
that is what has changed between 2.4 and 2.5 and could well change in the
future.

John

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Comment By: Raymond Hettinger (rhettinger)
Date: 2007-06-03 06:50

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I don't see an easy way to make this a defined behavior.

FWIW, the OP's code suggests that it makes a more specific test than it
does (since -0.0 == 0.0) so the test succeed when n is either -0.0 or 0.0. 
A quick fix in his code would be to eliminate the -0.0 from the code.  

def r(n):
    if n == 0.0:
        return 0.0
    return n

or more succinctly:

def r(n):
    return n or 0.0


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Comment By: Neal Norwitz (nnorwitz)
Date: 2007-06-01 05:49

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This is a regression from 2.4.  This seems to always have been undefined
behaviour.  It looks like it was the result of the compiler changes (code
is the same in both versions, but co_consts is diff):

Python 2.4.4c1 (#2, Oct 11 2006, 20:00:03) 
>>> def r(n):
...   if n == -0.0: n = 0.0
...   return n
... 
>>> r.func_code.co_consts
(None, 0.0)

Python 2.6a0 (trunk, May 30 2007, 21:02:18) 
>>> def r(n):
...  if n == -0.0: n = 0.0
...  return n
... 
>>> r.func_code.co_consts
(None, -0.0)


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