On 2010-01-08 07:48 AM, CELEN Erman wrote:
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.
Numeric.log10() will check to see if the errno was set to ERANGE. It does not
check if a floating point exception flag was set, which is tricky to do across
platforms. The newer numpy can do it because we've finally managed to implement
all of that platform-specific code, but the earlier Numeric does not.
Presumably, the msvc8 C runtime's implementation of log10() sets errno and the
msvc9 runtime's version does not.
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)
Python 2.6 changed the string representations of the special floating point
values inf and nan. Previously, the string representation was left up to the C
runtime, so they differed between platforms. Python 2.6 normalized the string
representation across all platforms. The underlying values are the same. What
version of Python are you using?
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
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