mikeash.com: just this guy, you know?

Posted at 2009-11-21 02:44 | RSS feed (Full text feed) | Blog Index
Next article: Friday Q&A 2009-11-27: Using Accessors in Init and Dealloc
Previous article: Deconstructing Apple's Copyright and Trademark Guidelines
Tags: cocoa hack objectivec pyobjc python
Friday Q&A 2009-11-20: Probing Cocoa With PyObjC
by Mike Ash  

It's another Friday and time for another Friday Q&A. This week, fellow Amoeba Jeff Johnson suggested talking about using Cocoa from the command line using Python and PyObjC.

I assume everybody reading this knows what Cocoa is, but may not know what the other two parts are:

PyObjC gets implicitly loaded whenever you load one of the Python modules that need it. Most system frameworks have a corresponding Python module. To load such a module, you can enter import FrameworkName at the Python command line. However, since Python supports namespaces, this requires putting FrameworkName. before any symbol inside that framework that you want to use. This is usually a good thing for "real" code, but if we're just going to experiment with things from the command line, it's better to avoid that. You can tell Python to import everything in the framework into the top-level namespace instead with from FrameworkName import *. Then you can use symbols from the framework directly. Example:
    >>> from Foundation import *
    >>> NSFileManager
    <objective-c class NSFileManager at 0x7fff7101eb18>
And then you can send messages to these classes using Python's C++/Java-ish syntax:
    >>> NSFileManager.defaultManager()
    <NSFileManager: 0x100257900>
PyObjC automatically translates common object types across, such as strings, so you can just write normal string literals in Python and have it work. To pass a parameter, you have to deal with the syntax mismatch, because Python has a single method name, whereas Objective-C interleaves method name components with parameters. To translate to Python, glue all of the method's components together, then replace the colons with underscores. Examples:
    >>> NSFileManager.defaultManager().displayNameAtPath_('/')
    >>> NSFileManager.defaultManager().fileAttributesAtPath_traverseLink_('/', True) 
        NSFileCreationDate = "2006-08-18 08:33:34 -0400";
        NSFileExtensionHidden = 0;
        NSFileGroupOwnerAccountID = 80;
        NSFileGroupOwnerAccountName = admin;
        NSFileModificationDate = "2009-11-16 23:58:45 -0500";
        NSFileOwnerAccountID = 0;
        NSFileOwnerAccountName = root;
        NSFilePosixPermissions = 1021;
        NSFileReferenceCount = 38;
        NSFileSize = 1360;
        NSFileSystemFileNumber = 2;
        NSFileSystemNumber = 234881026;
        NSFileType = NSFileTypeDirectory;
Note that, while basic Python objects like strings and dictionaries will get converted to their Cocoa equivalents when passing through the bridge, they are not instances of their Cocoa equivalents if they stay in Python-land. In other words, if you have a Python string, you can't just go and use NSString methods with it:
    >>> 'abc'.length()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    AttributeError: 'str' object has no attribute 'length'
Instead, you can do a simple pass into Cocoa first to get it to work:
    >>> NSString.stringWithString_('abc').length()
(Of course in this specific example, you could just use the built in Python way of getting the length, with len('abc').)

Cocoa methods that return NSError instances by reference get special treatment by PyObjC. Python cleanly supports returning multiple values from a method, but doesn't cleanly support return-by-reference, so PyObjC translates the NSError return by reference into a multiple return. You just assign two variables to the result of the method, and then pass None (Python's version of nil for the NSError argument. Example:

    >>> string, error = NSString.stringWithContentsOfFile_encoding_error_('/', NSUTF8StringEncoding, None)
    >>> string
    >>> error.description().encode('utf8')
    'Error Domain=NSCocoaErrorDomain Code=257 UserInfo=0x11994b610 "The file \xe2\x80\x9cFear\xe2\x80\x9d couldn\xe2\x80\x99t be opened because you don\xe2\x80\x99t have permission to view it." Underlying Error=(Error Domain=NSPOSIXErrorDomain Code=13 "The operation couldn\xe2\x80\x99t be completed. Permission denied")'
I have to engage in a bit of trickery to print the error object at the end because of the non-ASCII characters it contains. If I just try to print error directly, Python will complain that its description can't be converted to ASCII, so I have to manually get the description and convert it to UTF-8 for printing.

Arrays and Dictionaries
A Python array can be written like this:

    ['a', 'b', 'c']
And PyObjC will convert it to an NSArray as it crosses the bridge. This makes it trivial to pass arrays to Cocoa methods which take them:
    >>> data, error = NSPropertyListSerialization.dataFromPropertyList_format_errorDescription_(['hello', 'world'], NSPropertyListXMLFormat_v1_0, None)
    >>> NSString.alloc().initWithData_encoding_(data, NSUTF8StringEncoding)
    u'<?xml version="1.0" encoding="UTF-8"?>\n<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">\n<plist version="1.0">\n<array>\n\t<string>hello</string>\n\t<string>world</string>\n</array>\n</plist>\n'
(Note that PyObjC is smart enough to understand Cocoa memory management conventions and so will automatically clean up the string that I allocated on the last line, rather than requiring me to explicitly release it.)

Dictionaries are written like this:

    { 'key' : 'value', 'key2' : 'value2' }
And likewise they get translated across the bridge:
    >>> data, error = NSPropertyListSerialization.dataFromPropertyList_format_errorDescription_({ 'key' : 'value', 'key2' : 'value2' }, NSPropertyListXMLFormat_v1_0, None)
    >>> NSString.alloc().initWithData_encoding_(data, NSUTF8StringEncoding)         u'<?xml version="1.0" encoding="UTF-8"?>\n<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">\n<plist version="1.0">\n<dict>\n\t<key>key</key>\n\t<string>value</string>\n\t<key>key2</key>\n\t<string>value2</string>\n</dict>\n</plist>\n'

Custom Frameworks
A fun thing with PyObjC is that it's not limited to system frameworks. You can load your own frameworks! It's trivial to do, because you can do it exactly as you would do it in a Cocoa program: just use NSBundle to load the framework and start getting and manipulating classes. Here's an example of doing this with a system framework, but it works just the same for your own:

    >>> bundle = NSBundle.bundleWithPath_('/System/Library/Frameworks/WebKit.framework')
    >>> bundle.principalClass()
    <objective-c class WebPlaceholderModalWindow at 0x7fff7095cc40>
    >>> bundle.classNamed_('WebView').alloc().init()
    <WebView: 0x100212490>
Once you have framework classes and objects, you can manipulate them just as you would Cocoa classes and objects.

Note that on 10.6 and 64-bit capable machines, Python loads as 64-bit by default, so if your framework is 32-bit only then this won't work. You can load Python in 32-bit mode by starting it with the following command (assuming you use the default bash shell):


So far we've just been working with Foundation-like objects, but PyObjC supports AppKit as well:

    >>> from AppKit import *
And it's trivial to get a little window up on the screen from there:
    >>> NSApplicationLoad()
    >>> window = NSWindow.alloc().init()
    >>> window.makeKeyAndOrderFront_(None)
Where this really comes in handy is with things like NSImage. This lets you easily manipulate images using classes you already know. For example:
    >>> image = NSImage.alloc().initWithContentsOfFile_('brakes.jpg')
    >>> image
    <NSImage 0x1198b6f70 Size={483, 450} Reps=(
        "NSBitmapImageRep 0x1198bcfa0 Size={483, 450} ColorSpace=(not yet loaded) BPS=8 BPP=(not yet loaded) Pixels=483x450 Alpha=NO Planar=NO Format=(not yet loaded) CurrentBacking=nil (faulting) CGImageSource=0x1198bc220"
    >>> scaledImage = NSImage.alloc().initWithSize_((32, 32))
    >>> scaledImage.lockFocus()
    >>> image.drawInRect_fromRect_operation_fraction_(((0, 0), (32, 32)), NSZeroRect, NSCompositeCopy, 1.0)
    >>> scaledImage.unlockFocus()
    >>> scaledImage.TIFFRepresentation().writeToFile_atomically_('/tmp/brakes_thumb.tiff', True)

This article barely scratches the surface of what's possible with PyObjC, but it should get you started. You can manipulate QuickTime movies, put up windows, test frameworks, and even write full-blown applications. For further reading, check out the PyObjC documentation and the Python documentation. This is a valuable tool to have in your kit, whether for testing, rapid prototyping, or just trying things out.

That's it for this week. Come back in seven days for another exciting edition. Until then, send in your ideas for topics to cover. Friday Q&A is driven by your ideas, so send them in!

Did you enjoy this article? I'm selling whole books full of them! Volumes II and III are now out! They're available as ePub, PDF, print, and on iBooks and Kindle. Click here for more information.


Thanks, Mike! A lot of times I find myself writing source files or even creating Xcode projects just to test one or two methods calls, which is overkill. The ability to call Cocoa methods from the command line is quite handy.
Great post Mike!

I frequently find myself firing up Python to see if a particular API works the way I expect, or just to figure out something without needless code-compile-debug cycles.

Another useful addition is using the ipython shell (http://ipython.scipy.org/moin/) which adds tab completion, improved history, and some other niceties. This makes a very easy environment for testing out new ideas quickly.

Also it's important to note that PyObjC is unfortunately not garbage collection safe, so if you are working with a GC only framework this method is off the table. (See this thread from the PyObjC list: http://bit.ly/6GqADe).
Excellent post.

I have been toying with the same concept, but using MacRuby instead of PyObjC. I was able to get most of you examples running in MacRuby as well. This approach makes a lot of sense for prototyping and for exploring the System libraries.

Again, thanks for the post.
Thanks Mike for this article :) I had no idea it was so easy in the command line to get a Cocoa window on screen! I'd never used NSApplicationLoad() before because it specifically says in the docs that it's used from Carbon, so it was kind of a surprise to me that it's useful in PyObjC too, but it makes sense after thinking about it more.
Jason Foreman: Good point about GC, I always forget that exists.

Jose Vazquez: There are a lot of options here, and Python+PyObjC is only one. Ruby, Perl, Nu, F-Script, are all other reasonable choices. I do Python simply because it's what I know best.
Jason Foreman: Oops I forgot to mention, MacRuby is the exact opposite. To get my framework to work I had to recompile it with Garbage Collection enabled. I believe MacRuby uses the Obj-C Garbage Collector as it's own, so I has to use it.
Another thing that i really like about PyObjc is that with py2app you can make frameworks indistinguishable from "regular" ones for other programs, thus making plugin prototyping really easy!
Thanks for this post, the line of code below saved me. PyObjc is great but the documentation is a little thin on the ground!


I tried your example for using a custom framework. It works fine when I do it in the terminal like your example but when I try to do it in a script , my window controller will only display for 1 second. What am I missing that will hold the window open?


    from AppKit import *
    from Foundation import *
    import objc


    bundle = NSBundle.bundleWithPath_('/Library/Frameworks/MyFramework.framework')
    controller = bundle.classNamed_('MyController').alloc().initWithWindowNibName_('MyWindow')

Comments RSS feed for this page

Add your thoughts, post a comment:

Spam and off-topic posts will be deleted without notice. Culprits may be publicly humiliated at my sole discretion.

The Answer to the Ultimate Question of Life, the Universe, and Everything?
Formatting: <i> <b> <blockquote> <code>.
NOTE: Due to an increase in spam, URLs are forbidden! Please provide search terms or fragment your URLs so they don't look like URLs.
Code syntax highlighting thanks to Pygments.
Hosted at DigitalOcean.