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• Inaccurate Logarithm in Python

  - by Avihu Turzion

  I work daily with Python 2.4 at my company. I used the versatile logarithm function 'log' from the standard math library, and when I entered log(2**31, 2) it returned 31.000000000000004, which struck me as a bit odd. I did the same thing with other powers of 2, and it worked perfectly. I ran 'log10(2**31) / log10(2)' and I got a round 31.0 I tried running the same original function in Python 3.0.1, assuming that it was fixed in a more advanced version. Why does this happen? Is it possible that there are some inaccuracies in mathematical functions in Python?

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• python xml.dom.minidom.Attr question

  - by rudy

  Getting attributes using minidom in Python, one uses the "attributes" property. e.g. node.attributes["id"].value So if I have <a id="foo"></a>, that should give me "foo". node.attributes["id"] does not return the value of the named attribute, but an xml.dom.minidom.Attr instance. But looking at the help for Attr, by doing help('xml.dom.minidom.Attr'), nowhere is this magic "value" property mentioned. I like to learn APIs by looking at the type hierarchy, instance methods etc. Where did this "value" property come from?? Why is it not listed in the Attr class' page? The only data descriptors mentioned are isId, localName and schemaType. Its also not inherited from any superclasses. Since I'm new to Python, would some of the Python gurus enlighten?

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• Parallelism in Python

  - by fmark

  What are the options for achieving parallelism in Python? I want to perform a bunch of CPU bound calculations over some very large rasters, and would like to parallelise them. Coming from a C background, I am familiar with three approaches to parallelism: Message passing processes, possibly distributed across a cluster, e.g. MPI. Explicit shared memory parallelism, either using pthreads or fork(), pipe(), et. al Implicit shared memory parallelism, using OpenMP. Deciding on an approach to use is an exercise in trade-offs. In Python, what approaches are available and what are their characteristics? Is there a clusterable MPI clone? What are the preferred ways of achieving shared memory parallelism? I have heard reference to problems with the GIL, as well as references to tasklets. In short, what do I need to know about the different parallelization strategies in Python before choosing between them?

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• What Python based Dashboard options exist?

  - by stuartcw

  I want to create a Dashboard on each server to show it's health and the results of some daily processing. I plan to hook up shell scripts and Python programs to collect the data. Instead of writing a web-based interface, I thought it would be good to use a python based web dashboard that could render the results in various business user and manager friendly formats. What are my options to do this? I am primarily interested in Python RedHat Linux, but other platforms are interesting too. I'm also open to Perl and Ruby based solutions especially if the plugins can be language neutral.

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• How to prevent traffic to/from a slow Cassandra node using Python

  - by Sergio Ayestarán

  Intro: I have a Python application using a Cassandra 1.2.4 cluster with a replication factor of 3, all reads and writes are done with a consistency level of 2. To access the cluster I use the CQL library. The Cassandra cluster is running on rackspace's virtual servers. The problem: From time to time one of the nodes can become slower than usual, in this case I want to be able to detect this situation and prevent making requests to the slow node and if possible to stop using it at all (this should theoretically be possible since the RF is 3 and the CL is 2 for every single request). The questions: What's the best way of detecting the slow node from a Python application? Is there a way to stop using one of the Cassandra nodes from Python in this scenario without human intervention? Thanks in advance!

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• Check if key is pressed using python (a daemon in the background)

  - by Nazarius Kappertaal

  I've created a python script in which an event needs to be executed each time I press the Super (or WinKey) on my keyboard. How can one achieve this without the python process being "focused" - as it is running in the background waiting for the key to be pressed to execute the event? I've seen a lot of posts around the web showing me how to read input - but they have all required one to have the process "focused" and none have showed me how to capture the Super (or WinKey) using a python script. I'm running Ubuntu 9.10.

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• Named semaphores in Python?

  - by Boaz

  Hi, I have a script in python which uses a resource which can not be used by more than a certain amount of concurrent scripts running. Classically, this would be solved by a named semaphores but I can not find those in the documentation of the multiprocessing module or threading . Am I missing something or are named semaphores not implemented / exposed by Python? and more importantly, if the answer is no, what is the best way to emulate one? Thanks, Boaz PS. For reasons which are not so relevant to this question, I can not aggregate the task to a continuously running process/daemon or work with spawned processed - both of which, it seems, would have worked with the python API.

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• Query size of block device file in Python

  - by ??O?????

  Hello. I have a Python script that reads a file (typically from optical media) marking the unreadable sectors, to allow a re-attempt to read said unreadable sectors on a different optical reader. I discovered that my script does not work with block devices (e.g. /dev/sr0), in order to create a copy of the contained ISO9660/UDF filesystem, because os.stat().st_size is zero. The algorithm currently needs to know the filesize in advance; I can change that, but the issue (of knowing the block device size) remains, and it's not answered here, so I open this question. I am aware of the following two related SO questions: Determine the size of a block device (/proc/partitions, ioctl through ctypes) how to check file size in python? (about non-special files) Therefore, I'm asking: in Python, how can I get the file size of a block device file?

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• Paver 0.8.1 compatibility with python 2.6

  - by Bertrand

  Hi, Does anyone manage to bootstrap its development area using paver with python 2.6 ? I have install python 2.6, install paver with easy_install-2.6, everything looks fine. But when I try to launch the bootstrap method it raises an urllib2.HTTPError (: HTTP Error 404: Not Found) while trying to download http://pypi.python.org/packages/2.6/s/setuptools/setuptools-0.6c8-py2.6.egg. I have tryed to add the correct setuptools EGG file (which is 0.6c9) in the support-files directory, bootstrap.py find the EGG file, but doesn't seem to use it because it still try to download the 0.6c8 version which is no more available. Any ideas how to solve this issue ? Thanks in advance Bertrand

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• Python os.path.join

  - by Jim

  Hello, I am trying to learn python and am making a program that will output a script. I want to use os.path.join but am pretty confused (I know I am very bad at scripting/programming) See, according to the docs ( http://docs.python.org/library/os.path.html ) if I say os.path.join('c:', 'sourcedir') I get C:sourcedir as it's output. According to the docs, this is normal (right?) But when I use the copytree command, Python will output it the desired way, for example import shutil src = os.path.join('c:', 'src') dst = os.path.join('c':', 'dst') shutil.copytree(src, dst) Here is the error code I get WindowsError: [Error 3] The system cannot find the path specified: 'C:src/.' If I wrap the os.path.join with os.path.normpath I get the same error If this os.path.join can't be used this way, then I am confused as to its purpose According to the pages suggested by Stack Overflow, slashes should not be used in join--that is correct I assume? Thanks guys(girls) for your help

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• Python Pari Library?

  - by silinter

  Pari/GP is an excellent library for functions relating to number theory. The problem is that there doesn't seem to be an up to date wrapper for python anywhere around, (pari-python uses an old version of pari) and I'm wondering if anyone knows of some other library/wrapper that is similar to pari or one that uses pari. I'm aware of SAGE, but it's far too large for my needs. GMPY is excellent as well, but there are some intrinsic pari functions that I miss, and I'd much rather use python than the provided GP environment. NZMATH, mpmath, scipy and sympy were all taken into consideration as well. On a related note, does anyone have any suggestions on loading the pari dll itself and using the functions contained in it? I've tried to very little success, other than loading it and learning about function pointers.

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• Unicode identifiers in Python?

  - by viksit

  Hi all, I want to build a Python function that calculates, and would like to name my summation function S. In a similar fashion, would like to use ? for product, and so on. I was wondering if there was a way to name a python function in this fashion? def S (..): .. .. That is, does Python support unicode identifiers, and if so, could someone provide an example for it? Thanks! Original motivation for this was a piece of Clojure code I saw today that looks like, (defn entropy [X] (* -1 (S [i X] (* (p i) (log (p i)))))) where S is a macro defined as, (defmacro S ... ) and I thought that was pretty cool.

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• How to write a shell in Python

  - by panzi

  I've written a small console application that can perform certain tasks. The user interface is similar to things like version control systems or yum etc. So basically you can think of it as a domain specific language. Now I'd like to write a (bash like) shell that can execute and auto-complete this language and has a command history (so I do not have to load and save the quite large xml files on each command). In a nutshell I want something like ipython but not for executing python code but my own DSL. Are there any libraries that help me doing this? I see that there is a readline and rlcompleter module in python but its documentation seems to indicate that this is only for use with the python shell itself, or did I miss something there?

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• How does polymorphism work in Python?

  - by froadie

  I'm new to Python... and coming from a mostly Java background, if that accounts for anything. I'm trying to understand polymorphism in Python. Maybe the problem is that I'm expecting the concepts I already know to project into Python. But I put together the following test code: class animal(object): "empty animal class" class dog(animal): "empty dog class" myDog = dog() print myDog.__class__ is animal print myDog.__class__ is dog From the polymorphism I'm used to (e.g. java's instanceof), I would expect both of these statements to print true, as an instance of dog is an animal and also is a dog. But my output is: False True What am I missing?

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• java version of python-dateutil

  - by elhefe

  Python has a very handy package that can parse nearly any unambiguous date and provides helpful error messages on a parse failure, python-dateutil. Comparison to the SimpleDateFormat class is not favorable - AFAICT SimpleDateFormat can only handle one exact date format and the error messages have no granularity. I've looked through the Joda API but it appears Joda is the same way - only one explicit format can be parsed at a time. Is there any package or library that reproduces the python-dateutil behavior? Or am I missing something WRT Joda/SimpleDateFormat?

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• what are the advantages of C# over Python

  - by Matt

  I like Python mostly for the great portability and the ease of coding, but I was wondering, what are some of the advantages that C# has over Python? The reason I ask is that one of my friends runs a private server for an online game (UO), and he offered to make me a dev if I wanted, but the software for the server is all written in C#. I'd love to do this, but I don't really have time to do multiple languages, and I was just after a few more reasons to justify taking C# over Python to myself. I'm doing this all self-taught as a hobby, btw

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• lightweight cryptography toolkit(s) for c++ and python

  - by Joey

  Hi, I'm looking to do some basic encryption of server messages which'd be encrypted with C++ and decrypted using Python serverside. I was wondering if anyone knew if there were good solutions that were simpler or more lightweight than Keyczar. I see that supports both C++ and python, but would using Crypto++ and PyCrypto be simpler for a newbie that just wants to get something up and running for the time being? Or should I use Keyczar for python and Crypto++ for the C++ end? The C++ libraries seem to have dependencies to hundreds of files.

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• Python ValueError: not allowed to raise maximum limit

  - by Ricky Bobby

  I'm using python 2.7.2 on mac os 10.7.3 I'm doing a recursive algorithm in python with more than 50 000 recursion levels. I tried to increase the maximum recursion level to 1 000 000 but my python shell still exit after 18 000 recursion levels. I tried to increase the resources available : import resource resource.setrlimit(resource.RLIMIT_STACK, (2**29,-1)) sys.setrecursionlimit(10**6) and I get this error : Traceback (most recent call last): File "<pyshell#58>", line 1, in <module> resource.setrlimit(resource.RLIMIT_STACK,(2**29,-1)) ValueError: not allowed to raise maximum limit I don't know why I cannot raise the maximum limit ? thanks for your suggestions .

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• CentOS: revert python version back to original

  - by NP

  Hi all, I installed python 2.6 using the instructions here on CentOS 5.4. However I realized it was a bad move and I need to revert back to 2.4, which was there originally. Can anyone guide me on how to undo what I did here? In particular, I am not sure how to undo this: Configure ld to find your shared libs: $ cat /etc/ld.so.conf.d/opt-python2.5.conf /opt/python2.5/lib (hit enter) (hit ctrl-d to return to shell) $ ldconfig I tried removing the alias and the symlink and even re-aliasing python to /usr/bin/python, but when I try to install an RPM i get this error: error: Failed dependencies: libpython2.4.so.1.0 is needed by ... Thanks in advance.

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• Is it possible to install IDLE on Python 2.5 and Ubuntu

  - by broiyan

  Ubuntu Software Center offers versions of IDLE numbered 2.6 and upwards. If I have to use Python 2.5 (because of Google App Engine compatibility), will it be possible to still use IDLE? I am of course assuming that the there is some necessity for the IDLE version number to be the same as the Python version number. Presently, my invocations of IDLE fail because of _tkinter not being found. Likewise it is not found when I try to do an import _tkinter interactively. I then proceeded to install Tcl and Tk and python-tk and one of the IDLE versions being offered (none were labelled 2.5) but _tkinter still does not get imported.

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• Python - converting wide-char strings from a binary file to Python unicode strings...

  - by Mikesname

  It's been a long day and I'm a bit stumped. I'm reading a binary file that contains lots of wide-char strings and I want to dump these out as Python unicode strings. (To unpack the non-string data I'm using the struct module, but I don't how to do the same with the strings.) For example, reading the word "Series": myfile = open("test.lei", "rb") myfile.seek(44) data = myfile.read(12) # data is now 'S\x00e\x00r\x00i\x00e\x00s\x00' How can I encode that raw wide-char data as a Python string? Edit: I'm using Python 2.6

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• An simple Python extension in C

  - by celil

  I am trying to create a simple python extension module. I compiled the following code into a transit.so dynamic module #include <python2.6/Python.h> static PyObject* _print(PyObject* self, PyObject* args) { return Py_BuildValue("i", 10); } static PyMethodDef TransitMethods[] = { {"print", _print, METH_VARARGS, ""}, {NULL, NULL, 0, NULL} }; PyMODINIT_FUNC inittransit(void) { Py_InitModule("transit", TransitMethods); } However, trying to call this from python import transit transit.print() I obtain an error message File "test.py", line 2 transit.print() ^ SyntaxError: invalid syntax What's wrong with my code?

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• Python - which multi platform GUI framework to use?

  - by Iacopo

  Hi, I've written a Python GUI application and made it run on Linux, Windows and Mac. The framework I'm using is PyGtk. The final result is not really good: the application looks horrible on Mac, and it is not really 'native' neither on Windows. Furthermore on Mac the windows' behavior is sometimes wrong, with modal dialogs appearing below the main windows. I've explored other frameworks and I've found that a porting may be really hard: PyQt it is huge, difficult to install and to distribute under Mac; PyGui is poor: I need tables and tree views at least; wxPython state explicitly that you have to use a custom python build for Mac Tkinter: didn't check it, only have the impression that it is obsolete I know this question may appear similar to http://stackoverflow.com/questions/394039/which-python-gui-framework but in that thread the portability issue is not really taken in account. Windows and Mac are a must, since most of my user have that systems. Can you also suggest some books?

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• How to set parameters in Python zlib module

  - by fagricipni

  I want to write a Python program that makes PNG files. My big problem is with generating the CRC and the data in the IDAT chunk. Python 2.6.4 does have a zlib module, but there are extra settings needed. The PNG specification REQUIRES the IDAT data to be compressed with zlib's deflate method with a window size of 32768 bytes, but I can't find how to set those parameters in the Python zlib module. As for the CRC for each chunk, the zlib module documentation indicates that it contains a CRC function. I believe that calling that CRC function as crc32(data,-1) will generate the CRC that I need, though if necessary I can translate the C code given in the PNG specification. Note that I can generate the rest of the PNG file and the data that is to be compressed for the IDAT chunk, I just don't know how to properly compress the image data for the IDAT chunk after implementing the initial filtering step.

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• call multiple c++ functions in python using threads

  - by wiso

  Suppose I have a C(++) function taking an integer, and it is bound to (C)python with python api, so I can call it from python: import c_module c_module.f(10) now, I want to parallelize it. The problem is: how does the GIL work in this case? Suppose I have a queue of numbers to be processed, and some workers (threading.Thread) working in parallel, each of them calling c_module.f(number) where number is taken from a queue. The difference with the usual case, when GIL lock the interpreter, is that now you don't need the interpreter to evaluate c_module.f because it is compiled. So the question is: in this case the processing is really parallel?

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