Mailman 3 October 2000 - NumPy-Discussion

Re: Numpy-discussion digest, Vol 1 #112 - 8 msgs
by Greg Kochanski Oct. 12, 2000

Oct. 12, 2000

Python has *got* to ignore underflow exceptions. Otherwise, virtually any numeric calculation that subtracts numbers will fail for no good reason. Worse, it will fail in a very sporadic, data-dependent manner. With double precision numbers, the failures will be rare, but anyone doing computations with floats will see unwanted exceptions with noticeable frequency. I once did a numeric calculation (a SVD classifier) on a system with IEEE-754 underflow exceptions turned on, and I lost a good square inch of hair and a solid week of life because of it. At one point, the numbers were converted from doubles to floats, and about 1 in every 10,000,000 of the numbers were too small to represent as a float. So, in about 1 in ten runs (an hour each), the code would crash for no obvious reason. It was a debugging nightmare. If python catches underflows, I'm going back to FORTRAN. On less crucial topics, I'm strongly in favor of preserving NaN and Inf. If I want my code to crash when some computation goes awry, I'll use assert, and crash it myself. Any serious numeric code should be loaded with assertions (any code at all, in fact). Asking the interpreter to do it for you is a crude and blunt instrument. If I may ask, what kind of platforms are there where people do math where the hardware *won't* support IEEE-754?

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Re: [Numpy-discussion] Re: numpy, overflow, inf, ieee, and rich comparison
by L. Busby Oct. 11, 2000

Oct. 11, 2000

It was an optional module (fpectl) that got added into Python 1.5. Looks like it has been carried forward pretty much unchanged into the 2.0 release candidate. To use the facility, you need to build python with --with-fpectl, then identify "dangerous" (likely to generate SIGFPE) operations, and surround them with a pair of macros PyFPE_START_PROTECT and PyFPE_END_PROTECT. This has the effect of turning any SIGFPE into a Python exception. Start with Include/pyfpe.h, look for example usage in Objects/floatobject.c. Grep the python source for FPE_ to find the several places where these hooks are located. [ Paul Dubois wrote ] >About controlling floating-point behavior with Numpy: I think that somewhere >buried in the sources Lee Busby had us all set if we would just go through >the source and stick in some macro in the right places (this was maybe 3 >years ago, hence the accurate and detailed memory dump) but it was on the >todo list so long it outlived the death of the todo list. > >Anyway, my recollection is if we ever did it then we would have control when >things like overflow happen. But we haven't. My last experience of this sort >of thing was that it was completely a hardware-dependent thing, and you had >to find out from each manufacturer what their routine was and how to call it >or what compiler flag to use. > >Well, sorry to be so imprecise but I thought it worth mentioning that some >steps had been taken even if I don't remember what they were.

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Re: numpy, overflow, inf, ieee, and rich comparison
by Paul F. Dubois Oct. 11, 2000

Oct. 11, 2000

About controlling floating-point behavior with Numpy: I think that somewhere buried in the sources Lee Busby had us all set if we would just go through the source and stick in some macro in the right places (this was maybe 3 years ago, hence the accurate and detailed memory dump) but it was on the todo list so long it outlived the death of the todo list. Anyway, my recollection is if we ever did it then we would have control when things like overflow happen. But we haven't. My last experience of this sort of thing was that it was completely a hardware-dependent thing, and you had to find out from each manufacturer what their routine was and how to call it or what compiler flag to use. Well, sorry to be so imprecise but I thought it worth mentioning that some steps had been taken even if I don't remember what they were.

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version.py vs numeric_version.py
by Fred Yankowski Oct. 10, 2000

Oct. 10, 2000

I couldn't seem to download a prebuilt distribution of NumPy from SourceForge earlier today, so I connected via CVS and grabbed the latest code. I was eventually able to build and install it OK, but I had to copy Lib/version.py to Lib/numeric_version.py since the latter didn't exist but other code refers to it. Just a "heads up"... -- Fred Yankowski fred(a)OntoSys.com tel: +1.630.879.1312 Principal Consultant www.OntoSys.com fax: +1.630.879.1370 OntoSys, Inc 38W242 Deerpath Rd, Batavia, IL 60510, USA

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ANNOUNCE: PySymbolic - Doing Symbolics in Python
by Pearu Peterson Oct. 10, 2000

Oct. 10, 2000

Hi! I have started a project PySymbolic - "Doing Symbolics in Python" PySymbolic is a collection of tools for doing elementary symbolic calculations in Python. The project is in alpha stage. Current features include basic simplifications: *** removing redundant parenthesis: (a) -> a *** applying factorization: -(a) -> -a, +a -> a, ~(a) -> ~a *** collecting terms: n*a+b+m*a -> k*a+b (where k=n+m) *** collecting powers: a**c*b/a**d -> a**(c-d)*b *** cancellations: 0*a -> 0, a+0 -> 0, a**0 -> 1, 1*a -> a, a**1 -> a *** rational arithmetic: 3/4+2/6-1 -> 1/12 *** opening parenthesis: (a+b)*c -> a*c+b*c *** expanding integer powers: (a+b)**n -> a**n+a**(n-1)*b+... *** sorting terms in alphabethic order: a+c+b -> a+b+c *** (more to come) Projects homepage is http://cens.ioc.ee/projects/pysymbolic/ You can download the source from http://cens.ioc.ee/projects/pysymbolic/PySymbolic.tgz Regards, Pearu

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[ANNOUNCE] Ninth International Python Conference
by Paul F. Dubois Oct. 9, 2000

Oct. 9, 2000

CALL FOR PAPERS, POSTERS, AND PARTICIPATION Ninth International Python Conference March 5-8, 2001 Long Beach, California Web site: http://python9.org The 9th International Python Conference (Python 9) is a forum for Python users, software developers, and researchers to present current work, discuss future plans for the language and commercial applications, and learn about interesting uses of Python. The conference will consist of a day of tutorials, two days of refereed paper and application tracks including a Zope track and a multi-technology Python Applications Track, a developers' day, a small exhibition, demonstrations, and posters. Paper submission deadline: Monday, November 6, 2000 Notification of acceptance: Monday, December 11, 2000 Final papers due: Monday, January 15, 2001 Authors are invited to submit papers for the Refereed Paper Track that: * Present new and useful applications and tools that utilize Python * Describe the use of Python in large, mission-critical or unusual applications * Address practical programming problems, and provide lessons based on experience for Python programmers Also sought are poster presentations of interesting work in progress. Full information is available on the website, http://python9.org

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