[ python-Bugs-1678380 ] 0.0 and -0.0 identified, with surprising results

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Bugs item #1678380, was opened at 2007-03-11 18:16
Message generated for change (Comment added) made by loewis
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Category: Python Interpreter Core
Group: None
Status: Open
Resolution: Wont Fix
Priority: 5
Private: No
Submitted By: Mark Dickinson (marketdickinson)
Assigned to: Nobody/Anonymous (nobody)
Summary: 0.0 and -0.0 identified, with surprising results

Initial Comment:
The identification of -0.0 and 0.0 in scripts leads to some surprising
results.  In particular, code that behaves one way in the interpreter can behave differently in a script.  For example:

Python 2.6a0 (trunk:54183M, Mar  6 2007, 20:16:00) 
[GCC 4.0.1 (Apple Computer, Inc. build 5367)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from math import atan2;
>>> x = -0.
>>> y = 0.
>>> print atan2(y, -1.)
3.14159265359

But:

>>> exec("from math import atan2; x = -0.; y = 0.; print atan2(y, -1.)")
-3.14159265359

A simpler example:

>>> x, y = -0., 0.
>>> x, y
(-0.0, -0.0)
>>> id(x) == id(y)
True

But:

>>> x = -0.
>>> y = 0.
>>> x, y
(-0.0, 0.0)

This occurs both on SuSE Linux 9.3/i686 and on OS X 10.4.8/PowerPC.


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>Comment By: Martin v. Löwis (loewis)
Date: 2007-03-11 23:27

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marketdickinson, you should ask this question (is this really acceptable)
on python-dev. I find it perfectly acceptable. No program should rely on -0
and +0 being two different things (and thus also not relying on atan2
giving two different results).

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Comment By: Gabriel Genellina (gagenellina)
Date: 2007-03-11 21:10

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It appears to be a problem in the way compile.c handles literals.
See http://mail.python.org/pipermail/python-list/2007-March/430302.html

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Comment By: Mark Dickinson (marketdickinson)
Date: 2007-03-11 20:21

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May I beg for reconsideration.
Is the following really considered acceptable?

>>> x = -0.; atan2(0., -1)
-3.1415926535897931
>>> x = -(0.); atan2(0., -1)
3.1415926535897931
>>> atan2(0., -1)
3.1415926535897931

A single x = -0. at the start of a script can have
side effects several hundred lines later, even when
the variable x is never referred to again.  I guess
the advice should be:  "To avoid surprises, -0. should
never appear in any script."

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Comment By: Martin v. Löwis (loewis)
Date: 2007-03-11 19:35

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This is not a bug, at least not one that will be fixed. Details of the
floating-point can vary across platforms, and they may behave in suprising
ways in various contexts. Users shouldn't rely on Python differentiating
between -0 and +0.

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