Hi All,
My problem is that I've noticed a strange behavior in Python while handling
FPEs on Windows after switching compilers (msvc8 to msvc9) and I am trying to
find out how Python handles INF values to figure out where the problem might
be.
The problem appeared when we noticed that behavior of Numeric (older version of
NumPy) library has changed when dealing with Floating-Point Exceptions. We are
building our own slightly modified version of Python in-house (including
Numeric/NumPy) and when we switched from the msvc8 to msvc9 compiler, Numeric
started to return '-inf' as a result of 'Numeric.log10(0.0)' instead of rasing
an OverflowError. I know that Numeric is using umath instead of the math
library and since math.log10(0.0) is still raising an Error (which is
ValueError in 2.6 and OverflowError in 2.4, but still an error) I thought that
it might have something to do with how umath or Numeric handles the input or
return values of log functions.
Before hunting blindly I wanted to see how Python interprets INF results. I
built a simple module with only one function that causes an FPE with log10(0.0)
using C's math library and prints the results with printf. Then I imported that
module from python, called the function and printed the result in python too to
see if there are any differences between two values (which I found out that
there are)
Here is the that part of the code from my C Python module:
static PyObject *badNum(PyObject *self, PyObject *args)
{
int sts = 0; //Not used, just dummy
double a = 0.0;
double c = 0.0;
if (!PyArg_ParseTuple(args, "i", &sts)) //Dummy check, no use
return NULL;
disableFPEs();
c = log10(a); //Since FPEs disabled, should return '-1.#INF'
enableFPEs();
printf("C-Val: %f\n", c);
return Py_BuildValue("d", c);
}
After I build the module and imported it into Python, I called the function and
printed the return value in Python. The output was different in two versions of
Python:
Output1 (Module was built with msvc8 and run by python24)
C-Val: -1.#INF
Py-Val: -1.#INF
Output2 (Module was built with msvc9 and run by python26)
C-Val: -1.#INF
Py-Val: -inf
I now know that compiler change didn't cause any change in return value of
log10(0.0) at C level (both compilations resulted in '-1.#INF') so I am
thinking that somehow at somewhere in Python core the value is translated into
'-inf' and that might be the reason why Numeric/NumPy is missing the problem
and having issues raising OverflowError like it's supposed to.
I started debugging Python.exe (with my module loaded) in Visual Studio. While
tracing what's happening after the line "return Py_BuildValue("d", c);" I went
down to the "floatobject.c " file (which I thought is highly relevant to
floating-point value conversions) and in particular "format_float()" function
but since I am not that familiar with Python internals, I am having trouble
pinpointing the issue and the code where the "conversion" is happening and I
can't even be sure if I am looking at the right place.
I would appreciate if someone could help me out or at least could point me to
the right direction so that I can figure out the rest.
Thanks,
Best Regards,
Ali Erman CELEN
Platform Specialists / Porting
Dassault Systèmes I www.3ds.com <http://www.3ds.com/>
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