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Re: Python 2.5.1: Potential Bug in "print"

Mike MacFerrin wrote:
I'm a grad student in a Compiler Tools course, and (as part of the course) we're compiling a subset of the Python language down to C, and from there to x86 Assmebly. Our code has to pass a lengthy series of test-suites for correctness (comparing the output of our compiled C Code with that of native Python code), and I stumbled across what seems to be a quirky bug in Cygwin Python 2.5.1 (the most recent install).

To the point. If I run a simple two-line Python module:

print 0.0 print -0.0 gives the following output (as you'd expect):

0.0 0.0

Not what I would expect or want given that IEEE 754 supports negative zero.

However, if I remove the first line:

print -0.0

...I get...


It seems to matter whether I've previously called "print 0.0" in the module.

There is a bug in the Python optimizer <> and patch at <>.

No other "print <float>" commands make a difference.

It's not the printing that does it, it's the compilation. When a unit of code is compiled in Python 2.5, all the things that compare equal to 0.0 are collapsed. If you had another sign-sensitive function, you could call it and get the same results.

Obviously, that seems strange (and doesn't follow the Python language definition).

Where is this "Python language definition" and where does this behavior contradict it? I believe that this behavior is platform-specific and undefined for Python.

Run python and check for yourself if you wanna see it (from the shell it always prints "-0.0", but from a file it's inconsistent.

Not really. This is from the shell:

>>> print 0.0, -0.0
0.0 0.0

Either way, it should just spit out "0.0").

I disagree. In the future you should expect -0.0 to consistently spit out -0.0.

In the meantime, I'm a bit frustrated that my code fails certain tests at home on Cygwin (due to no error of my own),

The error is in the tests that expect 0.0 and -0.0 to produce the same results. Once this bug is fixed, I don't think they will on Linux anymore either.

Any suggestions or workarounds?

You can incorporate the patch I've linked, or you can get consistent results if you use -(0.0). Also, dealing with IEEE 754 negative zeros seems like something that would be useful in a compiler course so you should bring it up with your professor.

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