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• why the hell does x,y = zip(*zip(a,b)) work in Python?

  - by Mike Dewar

  OK I love Python's zip() function. Use it all the time, it's brilliant. Every now and again I want to do the opposite of zip(), think "I used to know how to do that", then google python unzip, then remember that one uses this magical * to unzip a zipped list of tuples. Like this: x = [1,2,3] y = [4,5,6] zipped = zip(x,y) unzipped_x, unzipped_y = zip(*zipped) unzipped_x Out[30]: (1, 2, 3) unzipped_y Out[31]: (4, 5, 6) What on earth is going on? What is that magical asterisk doing? Where else can it be applied and what other amazing awesome things in Python are so mysterious and hard to google?

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• side effect gotchas in python/numpy? horror stories and narrow escapes wanted

  - by shabbychef

  I am considering moving from Matlab to Python/numpy for data analysis and numerical simulations. I have used Matlab (and SML-NJ) for years, and am very comfortable in the functional environment without side effects (barring I/O), but am a little reluctant about the side effects in Python. Can people share their favorite gotchas regarding side effects, and if possible, how they got around them? As an example, I was a bit surprised when I tried the following code in Python: lofls = [[]] * 4 #an accident waiting to happen! lofls[0].append(7) #not what I was expecting... print lofls #gives [[7], [7], [7], [7]] #instead, I should have done this (I think) lofls = [[] for x in range(4)] lofls[0].append(7) #only appends to the first list print lofls #gives [[7], [], [], []] thanks in advance

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• python os.execvp() trying to display mysql tables gives 1049 error - Unknown database error.

  - by Hemanth Murthy

  I have a question related to mysql and python. This command works on the shell, but not when I use os.execvp() $./mysql -D test -e "show tables" +----------------+ | Tables_in_test | +----------------+ | sample | +----------------+ The corresponding piece of code in python would be def execute(): args = [] args.extend(sys.argv[1:]) args.extend([MYSQL, '-D test -e "show tables"']) print args os.execvp(args[0], args) child_pid = os.fork() if child_pid == 0: os.execvp(args[0], args) else: os.wait() The output of this is: [./mysql', '-D test -e "show tables"'] ERROR 1049 (42000): Unknown database ' test -e "show tables"' I am not sure if this is a problem with the python syntax or not. Also, the same command works with os.system() call. os.system(MYSQL + ' -D test -e "show tables"') Please let me know how to get this working. Thanks, Hemanth

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• Where can I find a professional image gallery built on a javascript framework?

  - by user278457

  I'm looking to find a galleria replacement, hopefully using jQuery but other javascript frameworks such as prototype or mootools are fine too. I used galleria a while back, and I need a similar product now. Unfortunately, the devkick.com domain seems to have disappeared in the meantime and I'm wary of using products that aren't actively maintained. I'm willing to pay up to $50 per site for licensing costs, if the product meets my needs. I'm specifically looking for a gallery with the following features: Every image in the gallery preloads asap, not as the user clicks "next" Minimalist default css to keep my subsequent styling headaches down, preferably a "darkroom" style by default, much as galleria looks Each element that constructs the image gallery should be simple and logical to reference with CSS As easy to install as adding a css class to a single unordered list No dependencies other than the core jQuery/other library, including "easing" and other effects must be optional Works on browsers back to IE6, Firefox 3, Safari (and iPhone), Chrome, Opera Has a javascript API that lets me trigger callback functions on common events such as "user clicks next" or "image loads" degrades gracefully without javascript, either displays images as a list, or just displays the first image in the list bonus: The gallery can display other content, such as video or external sites, like the modal boxes at shadowbox-js.com well documented minimal bandwidth requirement - .js file should be ~10kb minified bonus: The gallery source is hosted on a reliable CDN like google's bonus: Thumbnails for images do not appear until the main image has loaded bonus: includes ability to set parameters with JSON to change common behaviours, such as slide/fade transitions or automatic image switch every X seconds

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• Using ManualResetEvent to wait for multiple Image.ImageOpened events

  - by umlgorithm

  Dictionary<Image, ManualResetEvent> waitHandleMap = new Dictionary<Image, ManualResetEvent>(); List<Image> images = GetImagesWhichAreAlreadyInVisualTree(); foreach (var image in images) { image.Source = new BitmapImage(new Uri("some_valid_image_url")); waitHandleMap.Add(image, new ManualResetEvent(false)); image.ImageOpened += delegate { waitHandleMap[image].Set(); }; image.ImageFailed += delegate { waitHandleMap[image].Set(); }; } WaitHandle.WaitAll(waitHandleMap.Values.ToArray()); WaitHandle.WaitAll blocks the current UI thread, so ImageOpened/ImageFailed events would never get fired. Could you suggest me an easy workaround to wait for the multiple ui events?

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• Assemble an image browser side with JavaScript or Flash?

  - by Kris Walker

  Would it be possible to assemble an image on the browser by 'concatenating' other downloaded images together? The use case is this. The page will display 36 different tiles (small images). The user should be able to arrange those tiles into a 6 x 6 grid and save the resulting grid to disk as an image. The best solution would be to do it all in the browser without Flash. The next best solution would be to allow the user to create the grid in the browser with simple JavaScript drag and drop functionality and then send the coordinates to the server for image processing. The last solution would be to do it all in the browser with Flash. Is it even possible for Flash to create an image and then allow the user to save it from the browser? I am familiar with the Pixastic JavaScript library ( http://www.pixastic.com/ ), but it relies on getting image data to and from a canvas element which is not very well supported. What if I send the tile images to the browser as base64 encoded strings? Could I use JavaScript to create the 6 x 6 grid image? And if so, is there some way of allowing the user to get it onto disk without relying on the canvas element?

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• How can I make the Python logging output to be colored?

  - by airmind

  Some time ago I saw a Mono application with colored output, probably because of it's log system, because all the messages were standardized. Now, Python has the logging module, and it let you specify a lot of options or customize it entirely, so I'm imagining that something like that would be possible too with Python, however I could not find it anywhere. Is there any way to make the Python logging module to output in color? What I want is for error messages to appear in red, for instance. Debug messages in blue or yellow, and so on. Of course this would probably only work on Linux, with compatible terminals (most modern terminals are), but I could fallback to the original logging output if color is not supported. Any ideas?

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• Running Python code from Java program, shoudl i be doing this?

  - by Space Rocker

  i have a scenario where i draw a network and set all it's paraments on swing based gui, after that i have to translate this network into a python based script which another framework reads and realize this network in the form of virtual machines. As an example have look here: from mininet.topo import Topo, Node class MyTopo( Topo ): def *__init__*( self, enable_all = True ): super( MyTopo, self ).__init__() Host = 1 Switch = 2 self.add_node( Switch, Node( is_switch=True ) ) self.add_node( Host, Node( is_switch=False ) ) self.add_edge( Host, Switch ) self.enable_all() topos = { 'mytopo': ( lambda: MyTopo() ) } It simply connects a host to a switch and realize this topology on mininet framework. Now for now in order to realize the drawn network on java GUI here is what i am doing: I simply take the information from GUI and creates a new python file like the one above using java code and then run this file in mininet, which works fine somehow. I want to know, is this the correct and robust way how i am doing this or should i be looking further into java-python bridge like scenarios to be more effective or so as to say more professional.

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• Is Python appropriate for algorithms focused on scientific computing?

  - by gmatt

  My interests in programming lie mainly in algorithms, and lately I have seen many reputable researchers write a lot of their code in python. How easy and convenient is python for scientific computing? Does it have a library of algorithms that compares to matlab's? Is Python a scripting language or does it compile? Is it a great language for prototyping an algorithm? How long would it take me to learn enough of it to be productive provided I know C well and OO programming somewhat? Is it OO based? Sorry for the condensed format of questions, but I'm very curious and was hoping a more experienced programmer could help me out.

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• Can I turn off implicit Python unicode conversions to find my mixed-strings bugs?

  - by Tal Weiss

  When profiling our code I was surprised to find millions of calls to C:\Python26\lib\encodings\utf_8.py:15(decode) I started debugging and found that across our code base there are many small bugs, usually comparing a string to a unicode or adding a sting and a unicode. Python graciously decodes the strings and performs the following operations in unicode. How kind. But expensive! I am fluent in unicode, having read Joel Spolsky and Dive Into Python... I try to keep our code internals in unicode only. My question - can I turn off this pythonic nice-guy behavior? At least until I find all these bugs and fix them (usually by adding a u'u')? Some of them are extremely hard to find (a variable that is sometimes a string...). Python 2.6.5 (and I can't switch to 3.x).

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• How can I consume a WSDL (SOAP) web service in Python?

  - by DavidM

  I want to use a WSDL SOAP based web service in Python. I have looked at the Dive Into Python code but the SOAPpy module does not work under Python 2.5. I have tried using suds which works partly, but breaks with certain types (suds.TypeNotFound: Type not found: 'item'). I have also looked at Client but this does not appear to support WSDL. And I have looked at ZSI but it looks very complex. Does anyone have any sample code for it? The WSDL is https://ws.pingdom.com/soap/PingdomAPI.wsdl and works fine with the PHP 5 SOAP client.

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• How do you make Python wait so that you can read the output?

  - by anonnoir

  I've always been a heavy user of Notepad2, as it is fast, feature-rich, and supports syntax highlighting. Recently I've been using it for Python. My problem: when I finish editing a certain Python source code, and try to launch it, the screen disappears before I can see the output pop up. Is there any way for me to make the results wait so that I can read it, short of using an input() or time-delay function? Otherwise I'd have to use IDLE, because of the output that stops for you to read. (My apologies if this question is a silly one, but I'm very new at Python and programming in general.)

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• C# Image.Clone to byte[] causes EDIT.COM to open on Windows XP

  - by JayDial

  It appears that cloning a Image and converting it to a byte array is causing EDIT.COM to open up on Windows XP machines. This does not happen on a Windows 7 machine. The application is a C# .NET 2.0 application. Does anyone have any idea why this may be happening? Here is my Image conversion code: public static byte[] CovertImageToByteArray(Image imageToConvert) { imageToConvert.Clone() as Image; if(clone == null) return null; imageToConvert.Dispose(); byte[] imageByteArray; using (MemoryStream ms = new MemoryStream()) { clone.Save(ms, clone.RawFormat); imageByteArray = ms.ToArray(); } return imageByteArray; } public static Image ConvertByteArrayToImage(byte[] imageByteArray, ImageFormat formatOfImage) { Image image; using ( MemoryStream ms = new MemoryStream(imageByteArray, 0, imageByteArray.Length)) { ms.Write(imageByteArray, 0, imageByteArray.Length); image = Image.FromStream(ms, true); } return image; } Thanks Justin

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• Can you make a python script behave differently when imported than when run directly?

  - by futuraprime

  I often have to write data parsing scripts, and I'd like to be able to run them in two different ways: as a module and as a standalone script. So, for example: def parseData(filename): # data parsing code here return data def HypotheticalCommandLineOnlyHappyMagicFunction(): print json.dumps(parseData(sys.argv[1]), indent=4) the idea here being that in another python script I can call import dataparser and have access to dataParser.parseData in my script, or on the command line I can just run python dataparser.py and it would run my HypotheticalCommandLineOnlyHappyMagicFunction and shunt the data as json to stdout. Is there a way to do this in python?

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• Image animation problem in silverlight

  - by Jak

  Hi followed " http://www.switchonthecode.com/tutorials/silverlight-3-tutorial-planeprojection-and-perspective-3d#comment-4688 ".. the animation is working fine. I am new to silver light. when i use dynamic image from xml instead of static image as in tutorial,.. it is not working fine, please help me on this. i used list box.. for this animation effect do i need to change listbox to some other arrangement ? if your answer yes means, pls give me some sample code. Thanks in advance. Xaml code: <ListBox Name="listBox1"> <ListBox.ItemTemplate> <DataTemplate> <StackPanel> <Image Source="{Binding imgurl}" HorizontalAlignment="Left" Name="image1" Stretch="Fill" VerticalAlignment="Top" MouseLeftButtonUp="FlipImage" /> </StackPanel> </DataTemplate> </ListBox.ItemTemplate> </ListBox> My C# code: //getting image URL from xml XElement xmlads = XElement.Parse(e.Result); //i bind the url in to listBox listBox1.ItemsSource = from ads in xmlads.Descendants("ad") select new zestItem { imgurl = ads.Element("picture").Value }; public class zestItem { public string imgurl { get; set; } } private int _zIndex = 10; private void FlipImage(object sender, MouseButtonEventArgs e) { Image image = sender as Image; // Make sure the image is on top of all other images. image.SetValue(Canvas.ZIndexProperty, _zIndex++); // Create the storyboard. Storyboard flip = new Storyboard(); // Create animation and set the duration to 1 second. DoubleAnimation animation = new DoubleAnimation() { Duration = new TimeSpan(0, 0, 1) }; // Add the animation to the storyboard. flip.Children.Add(animation); // Create a projection for the image if it doesn't have one. if (image.Projection == null) { // Set the center of rotation to -0.01, which will put a little space // between the images when they're flipped. image.Projection = new PlaneProjection() { CenterOfRotationX = -0.01 }; } PlaneProjection projection = image.Projection as PlaneProjection; // Set the from and to properties based on the current flip direction of // the image. if (projection.RotationY == 0) { animation.To = 180; } else { animation.From = 180; animation.To = 0; } // Tell the animation to animation the image's PlaneProjection object. Storyboard.SetTarget(animation, projection); // Tell the animation to animation the RotationYProperty. Storyboard.SetTargetProperty(animation, new PropertyPath(PlaneProjection.RotationYProperty)); flip.Begin(); }

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• How do I write raw binary data in Python?

  - by Chris B.

  I've got a Python program that stores and writes data to a file. The data is raw binary data, stored internally as str. I'm writing it out through a utf-8 codec. However, I get UnicodeDecodeError: 'charmap' codec can't decode byte 0x8d in position 25: character maps to <undefined> in the cp1252.py file. This looks to me like Python is trying to interpret the data using the default code page. But it doesn't have a default code page. That's why I'm using str, not unicode. I guess my questions are: How do I represent raw binary data in memory, in Python? When I'm writing raw binary data out through a codec, how do I encode/unencode it?

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• Implementation of a C pre-processor in Python or JavaScript?

  - by grrussel

  Is there a known implementation of the C pre-processor tool implemented either in Python or JavaScript? I am looking for a way to robustly pre-process C (and C like) source code and want to be able to process, for example, conditional compilation and macros without invoking an external CPP tool or native code library. Another potential use case is pre-processing within a web application, within the web browser. So far, I have found implementations in Java, Perl, and of course, C and C again. It may be plausible to use one of the C to JavaScript compilers now becoming available. The PLY (Python Lex and Yacc) tools include a cpp implemented in Python.

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• Python wrapper for Google Maps making Google App Engine crash?

  - by user1679332

  I am currently trying to build a web app using Google App Engine that will involve using the Google Maps API. Since I am coding in python, I tried importing the python wrapper for Google Maps (found here); however, performing the import will cause my web app to crash. Are there any suggestions for how I can fix this problem? I'm guessing the crash might have something to do with the fact that I need to incorporate the google maps python wrapper into my application, but how do I go about doing that? Thanks!

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• Is it faster to loop through a Python set of number or a set of letters?

  - by Scott Bartell

  Is it faster to loop through a Python set of numbers or a Python set of letters given that each set is the exact same length and each item within each set is the same length? Why? I would think that there would be a difference because letters have more possible characters [a-zA-Z] than numbers [0-9] and therefor would be more 'random' and likely affect the hashing to some extent. numbers = set([00000,00001,00002,00003,00004,00005, ... 99999]) letters = set(['aaaaa','aaaab','aaaac','aaaad', ... 'aaabZZ']) # this is just an example, it does not actually end here for item in numbers: do_something() for item in letters: do_something() where len(numbers) == len(letters) Update: I am interested in Python's specific hashing algorithm and what happens behind the scenes with this implementation.

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• Is there a way to convert code to a string and vice versa in Python?

  - by Dragos Toader

  The original question was: Is there a way to declare macros in Python as they are declared in C: #define OBJWITHSIZE(_x) (sizeof _x)/(sizeof _x[0]) Here's what I'm trying to find out: Is there a way to avoid code duplication in Python? In one part of a program I'm writing, I have a function: def replaceProgramFilesPath(filenameBr): def getProgramFilesPath(): import os return os.environ.get("PROGRAMFILES") + chr(92) return filenameBr.replace("<ProgramFilesPath>",getProgramFilesPath() ) In another part, I've got this code embedded in a string that will later be output to a python file that will itself be run: """ def replaceProgramFilesPath(filenameBr): def getProgramFilesPath(): import os return os.environ.get("PROGRAMFILES") + chr(92) return filenameBr.replace("<ProgramFilesPath>",getProgramFilesPath() ) """ How can I build a "macro" that will avoid this duplication?

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• Is there an easy way in Python to wait until certain condition is true?

  - by Checkers

  I need to wait in a script until a certain number of conditions become true? I know I can roll my own eventing using condition variables and friends, but I don't want to go through all the trouble of implementing it, since some object property changes come from external thread in a wrapped C++ library (Boost.Python), so I can't just hijack __setattr__ in a class and put a condition variable there, which leaves me with either trying to create and signal a Python condition variable from C++, or wrap a native one and wait on it in Python, both of which sound fiddly, needlessly complicated and boring. Is there an easier way to do it, barring continuous polling of the condition? Ideally it would be along the lines of res = wait_until(lambda: some_predicate, timeout) if (not res): print 'timed out'

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• Why would an image (the Mandelbrot) be skewed and wrap around?

  - by Sean D

  So I just wrote a little snippet to generate the Mandelbrot fractal and imagine my surprise when it came out all ugly and skewed (as you can see at the bottom). I'd appreciate a point in the direction of why this would even happen. It's a learning experience and I'm not looking for anyone to do it for me, but I'm kinda at a dead end debugging it. The offending generation code is: module Mandelbrot where import Complex import Image main = writeFile "mb.ppm" $ imageMB 1000 mandelbrotPixel x y = mb (x:+y) (0:+0) 0 mb c x iter | magnitude x > 2 = iter | iter >= 255 = 255 | otherwise = mb c (c+q^2) (iter+1) where q = x --Mandelbrot --q = (abs.realPart $ x) :+ (abs.imagPart $ x) --Burning Ship argandPlane x0 x1 y0 y1 width height = [(x,y)| y<-[y1,(y1-dy)..y0], --traverse from x<-[x0,(x0+dx)..x1]] --top-left to bottom-right where dx = (x1 - x0)/width dy = (y1 - y0)/height drawPicture :: (a->b->c)->(c->Colour)->[(a,b)]->Image drawPicture function colourFunction plane = map (colourFunction.uncurry function) plane imageMB s = createPPM s s $ drawPicture mandelbrotPixel (\x->[x,x,x]) $ argandPlane (-1.8) (-1.7) (0.02) 0.055 s' s' where s' = fromIntegral s And the image code (which I'm fairly confident in) is: module Image where type Colour = [Int] type Image = [Colour] createPPM :: Int -> Int -> Image -> String createPPM w h i = concat ["P3 ", show w, " ", show h, " 255\n", unlines.map (unwords.map show) $ i]

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• Has anyone ever had OpenCV to work with Python on MacOS?

  - by ?????

  I've been trying on and off for the past 6 months to get OpenCV to work with Python on MacOS. Every time there's a new release, I try again and fail. I've tried both 64-bit and 32-bit, and both the xcode gcc and gcc installed via macports. I just spend the past two days on it, hopeful that the latest OpenCV release, that appears to include Python support directly would work. It doesn't. I've also tried and failed to use this: http://code.google.com/p/pyopencv/ I've been using OpenCV with C++ or Microsoft C++/CLI for the past few years, but I'd love to use it with Python on a Mac because that is my primary development environment. I'd love to hear from anyone who's actually done this.

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• Python HTTPSConnection.close() does not appear to close the connection?

  - by Dave

  I'm not sure if this is a bug or if I'm just doing something wrong. If I were to do an HTTP connection like this: import httplib http_connection = httplib.HTTPConnection("192.168.192.196") http_connection.request("GET", "/") http_connection.sock.settimeout(20) response = http_connection.getresponse() data = response.read() http_connection.close() Then at a DOS prompt, I do this: netstat -ano | find /i "192.168.192.196:80" | find /i "ESTABLISHED" I get nothing. However, if I do the same thing, but change it to an HTTPSConnection: import httplib http_connection = httplib.HTTPSConnection("192.168.192.196") http_connection.request("GET", "/") http_connection.sock.settimeout(20) response = http_connection.getresponse() data = response.read() http_connection.close() Then do this: netstat -ano | find /i "192.168.192.196:443" | find /i "ESTABLISHED" I will actually see that the connection remains established until I actually ^Z out of the Python shell. This is happening in one of the applications I'm responsible for. Python isn't actually hanging there - it's simply leaving the connection open. Am I doing something wrong here? Do I need extra code to close the HTTPS connection? This is Python 2.6.4, btw.

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• What c# equivalent encoding does Python's hash.digest() use ?

  - by The_AlienCoder

  I am trying to port a python program to c#. Here is the line that's supposed to be a walkthrough but is currently tormenting me: hash = hashlib.md5(inputstring).digest() After generating a similar MD5 hash in c# It is absolutely vital that I create a similar hash string as the original python program or my whole application will fail. My confusion lies in which encoding to use when converting to string in c# i.e ?Encoding enc = new ?Encoding(); string Hash =enc.GetString(HashBytes); //HashBytes is my generated hash Because I am unable to create two similar hashes when using Encoding.Default i.e string Hash = Encoding.Default.GetString(HashBytes); So I'm thinking knowing the deafult hash.digest() encoding for python would help

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