🌐 Mailman 3 July 2003 - Python-list - python.org

Python speed vs csharp
by Mike Aug. 6, 2003

Aug. 6, 2003

Bear with me: this post is moderately long, but I hope it is relatively succinct. I've been using Python for several years as a behavioral modeling tool for the circuits I design. So far, it's been a good trade-off: compiled C++ would run faster, but the development time of Python is so much faster, and the resulting code is so much more reliable after the first pass, that I've never been tempted to return to C++. Every time I think stupid thoughts like, "I'll bet I could do this in C++," I get out my copy of Scott Meyers' "Effecive C++," and I'm quickly reminded why it's better to stick with Python (Meyers is a very good author, but points out lots of quirks and pitfalls with C++ that I keep thinking that I shouldn't have to worry about, much less try to remember). Even though Python is wonderful in that regard, there are problems. Here's the chunk of code that I'm spending most of my time executing: # Rational approximation for erfc(x) (Abramowitz & Stegun, Sec. 7.1.26) # Fifth order approximation. |error| <= 1.5e-7 for all x # def erfc( x ): p = 0.3275911 a1 = 0.254829592 a2 = -0.284496736 a3 = 1.421413741 a4 = -1.453152027 a5 = 1.061405429 t = 1.0 / (1.0 + p*float(x)) erfcx = ( (a1 + (a2 + (a3 + (a4 + a5*t)*t)*t)*t)*t ) * math.exp(-(x**2)) return erfcx This is an error function approximation, which gets called around 1.5 billion times during the simulation, and takes around 3500 seconds (just under an hour) to complete. While trying to speed things up, I created a simple test case with the code above and a main function to call it 10 million times. The code takes roughly 210 seconds to run. The current execution time is acceptable, but I need to increase the complexity of the simulation, and will need to increase the number of data points by around 20X, to roughly 30 billion. This will increase the simulation time to over a day. Since the test case code was fairly small, I translated it to C and ran it. The C code runs in approximately 7.5 seconds. That's compelling, but C isn't: part of my simulation includes a parser to read an input file. I put that together in a few minutes in Python, but there are no corresponding string or regex libraries with my C compiler, so converting my Python code would take far more time than I'd save during the resulting simulations. On a lark, I grabbed the Mono C# compiler, and converted my test case to C#. Here's the corresponding erfc code: public static double erfc( double x ) { double p, a1, a2, a3, a4, a5; double t, erfcx; p = 0.3275911; a1 = 0.254829592; a2 = -0.284496736; a3 = 1.421413741; a4 = -1.453152027; a5 = 1.061405429; t = 1.0 / (1.0 + p*x); erfcx = ( (a1 + (a2 + (a3 + (a4 + a5*t)*t)*t)*t)*t ) * Math.Exp(-Math.Pow(x,2.0)); return erfcx; } Surprisingly (to me, at least), this code executed 10 million iterations in 8.5 seconds - only slightly slower than the compiled C code. My first question is, why is the Python code, at 210 seconds, so much slower? My second question is, is there anything that can be done to get Python's speed close to the speed of C#? -- Mike --

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Re: Python vs PHP
by Bruno Desthuilliers Aug. 5, 2003

Aug. 5, 2003

Ian Bicking wrote: > (snip) > What you should remember when moving away from PHP, is that you no > longer have to use the server-page metaphor -- most of your application > logic should be in plain Python modules. Then you'll use your PSP (or > similar) page to present that code and respond to user input. Funny... when using PHP, I personnally write the logic in separate modules, and keep the presentation in 'server pages' that call the modules and format/print the result... I don't see why coding in PHP should be that different from coding in Python (except that one of them is a far better language whatsoever... Too bad I can't afford Python-powered hosting) Bruno

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chaco setup
by Robin Becker Aug. 5, 2003

Aug. 5, 2003

I'm having trouble building chaco from source with Python- 2.2.3/Win2kSP4. I have Numeric 23.0 installed so assumed that it might not work using the win32 installer so am trying to build from chaco- 0.1.0_alpha_37.776.zip. I get a setup.py error ImportError: No module named build_agg. Do I need to install some extra software prior to building chaco? -- Robin Becker

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HTML DOM parser?
by Paul Rubin Aug. 4, 2003

Aug. 4, 2003

Is there an HTML DOM parser available for Python? Preferably one that does a reasonable job with the crappy HTML out there on real web pages, that doesn't get upset about unterminated tables and stuff like that. Many extra points if it understands Javascript. Application is a screen scraping web robot. Thanks.

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Bewildered graphs
by Mark Fenbers Aug. 4, 2003

Aug. 4, 2003

I am investigating Python for the sake of ultimately generating hydrographs (such as this: http://ahps.erh.noaa.gov/iln/ahps/RiverDat/gifs/prgo1.png) on-the-fly from a web server cluster. I have not used Python previously and do not yet know if Python is very useful for this or if I am wasting my time. Quite frankly, I am a little bewildered. Not only is there Python, but there are many extention modules which cloud up my view as to what I will need. There's Scientific Python, which sounds promising, but there's also SciPy which in itself has gnuplot, xplt and plt modules. I know enough about gnuplot to know that it won't meet my needs because I need to be able to shade regions above certain values such as done in yellow on the example hydrograph (the link above). It also doesn't have many font options or the ability to place an image such as the NOAA logo. Can someone kindly guide me as to what I would most likely need to replicate the graph shown via the link above? Mark

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Dict to "flat" list of (key,value)
by Nicolas Girard Aug. 4, 2003

Aug. 4, 2003

Hi, Forgive me if the answer is trivial, but could you tell me how to achieve the following: {k1:[v1,v2],k2:v3,...} --> [[k1,v1],[k1,v2],[k2,v3],...] The subtle point (at least to me) is to "flatten" values that are lists. Thanks in advance, Nicolas ----== Posted via Newsfeed.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeed.com The #1 Newsgroup Service in the World! >100,000 Newsgroups ---= 19 East/West-Coast Specialized Servers - Total Privacy via Encryption =---

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Python debugger
by Andrew Chalk Aug. 3, 2003

Aug. 3, 2003

I am sure that it has been asked before but could someone tell a newbie whether there is a Python debugger out there? In case it is relevant, i am using v2.2 on Windows. Many thanks.

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Re: looking for design pattern name
by jjl＠pobox.com Aug. 3, 2003

Aug. 3, 2003

"Andrew Dalke" <adalke(a)mindspring.com> writes: > Raymond Hettinger: > > What happens in Bond() if O1 and O2 have different parents? > > Raises an exception - TypeError seems best. Here's the code [...] Yuck. Doesn't the standard library reserve TypeError for occasions where the, um, type of an argument is not appropriate? 'Type' does generally means the interface rather than type(object), but that's quite different to what you're checking. Checking whether atoms have different parents surely isn't a type check. Why not ValueError or your own exception (possibly derived from ValueError)? [.........] > Err, I tangented, didn't I. [...] "Verbing weirds language", to quote Calvin. :-) John

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Secure FTP in Python?
by d z e k y l Aug. 3, 2003

Aug. 3, 2003

Hello, I'd like to write a small backup utility in Python, which would take advantage of Secure FTP to upload backup archives. Is there a library implementing SFTP in Python (something like ftplib)? Or is there some other (noncomplicated) way how secure uploading can be implemented in Python? Thank you for advice, Michal

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Re: SSH and Windows
by Peter Hansen Aug. 2, 2003

Aug. 2, 2003

Isaac Raway wrote: > > Hello. I'm writing a Python program that connects to servers through > telnetlib to execute a few commands. I've discovered that some of the > servers that I have to connect to with this program run only SSH, so I > need to add support for SSH to the program. I'm looking for a library > that behaves similarly to telnetlib for SSH connections. Does anyone > know of one? I was going to try using pexpect to control the Windows > telnet command, but pexpect only works on Unix. If it's just to execute a few commands, consider downloading PLink (from the PuTTY site) and use it via os.system(). It works well. -Peter

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