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Part 1 - Creating a basic CocoaBindings app with PyObjC

Python provides a clean, easy to learn, easy to develop in programming language. Cocoa provides a rich, easy to program framework for building applications on Mac OS X. Given that development should be easy, add PyObjC into the mix and the richness of Cocoa is available naturally from within Python. Now to see how easy it is to write a very simple application.

My first steps with attempting to create a Cocoa application with Python involved working through the tutorial on the PyObjC site. This is a very good place to start if you have an understanding of Cocoa. Some understanding of Python will also be helpful.

Note: If you are running on Tiger, you’ll need the latest version.

The most important thing to learn is how to build one of the sample applications:

$ python py2app -A

This will create an application bundle to run in the dist directory, using symbolic links to the code. As your source code is linked rather than copied, any changes you make are reflected in the bundle so you don’t need to re-deploy. Just re-run the application and you will be using the new code.

It is worth spending some time playing with some of the samples. The PyObjC team have done a very good job of putting together a wide range of examples. See /Developer/Python/PyObjC/Examples on your system. Try changing the files around a bit, I find it a useful way to learn when I break things by trying to add new stuff.

The first step in making your own application is to create a build script. This contains the information that py2app will use to generate your Cocoa application. A basic version looks like:

from distutils.core import setup
import py2app


The app field is the name of the python file that will be called first. In some of the examples this is called and in others it is the name of the application. I don’t like underscores so much, so I’m going with

The data_files field contains a list of directories in which data files are stored. For the moment, the only data file that you need to consider is the NIB files. These specify the interface for your application and are created by Interface Builder. If you wanted to load images and various other data, your entry might look like:

data_files=['English.lproj', 'data', 'images']

The next step is to create the main file, in this case,

from PyObjCTools import AppHelper
import DateListDelegate


The first line imports the AppHelper which is then called to run the main event loop for the application. This does a range of interesting Objective C / Cocoa interactions behind the scene, but thanks to PyObjC, we don’t need to worry about that.

The second line imports the python file that we will now create,

from PyObjCTools import NibClassBuilder


class DateListDelegate(NibClassBuilder.AutoBaseClass):
  def items(self):
    return []

There are two important things that happen in this script. Firstly, our user interface is loaded using the NibClassBuilder. Secondly, we create a class to use in our interface which is going to provide us with a list of data. For the moment, we simply return the empty list [].

I’m not going to explain in detail how to use Interface Builder. What I am going to do is to work through what is required to make our simple application fly.

From Interface Builder, create a new Cocoa Application and store it as MainMenu in a sub-folder called English.lproj. You may need to create this folder if you haven’t previously.

Next, add an NSTableView to your Window. This control is available in the Cocoa-Data palette. The palette(s) are the bunch of controls that should turn up in the top-right of your screen.

Cocoa Data palette

If you select Test Interface from the File menu in Interface Builder, you should see your window with a completely empty box. Not particularly compelling yet, but one step at a time.

You can now run your application from the command line. Save the Nib and build a linked version. It is easier to use linked for now as we will be changing the python files a bit and it saves re-deploying each time you run the application.

$ python py2app -A% open dist/

We now need to let Interface Builder know how to talk to the Python class we created earlier in In the MainMenu window, jump to the Classes tab and scroll left lots until you see the NSObject class. With that selected, from the Classes menu chose Subclass NSObject, and name your new class: DateListDelegate.

It is important that this class name exactly matches the class name that you create in Python, as some Cocoa magic happens behind the scenes to glue this all together.

With your new class highlighted, choose the Instantiate DateListDelegate option from the Classes menu. You should now have a funky blue cube in the Instances tab of the MainMenu window.

We need to have our class managed for us, so we’ll connect it to the application. To do this hold down Control then click and drag from the File’s Owner instance to the DateListDelegate instance. You should see the Outlets tab of the Connections view of the Inspector. From the Inspector, select the Connect button. This will connect our delegate class with the delegate property of the main application.

In order to join together the control on the Window with our Delegate class, we need a Controller object. This is where CocoaBindings really help us out, as you just need to drag a NSArrayController from the Cocoa-Controllers palette to the Instances tab of the MainMenu window.

Building blocks

Now we get up to the fun part, gluing it all together!

Firstly, we need some test data to play with. In a future article, I’m going to look at populating this from an external source, but for now we are just looking at the user interface, so we can fake it.

Open up the file and update the items method to look something like:

def items(self):
  return [
    {'date': 1980, 'name': 'John Doe'},
    {'date': 1970, 'name': 'Amanda Smith'},
    {'date': 1960, 'name': 'Bill Branson'}]

The data representation is how the Key-Value pairs, used by CocoaBindings, are constructed in Python. This is something that will be covered in more detail when we look at generating the structure from real data. If you want more information, see the Key-Value Coding section of the Apple developer site, or look through the examples in PyObjC.

The keys in our example are date and name, so we’d better let our Controller know that these are the fields it is going to be looking after.

In the Attributes section of the Inspector (Tools->Show Inspector) with the NSArrayController selected, there is an Add button. Add two keys, and rename them to the names that we have assigned in the data we are providing, date and name.

Not much use, unless our controller knows how to access our data. This is achieved from the Bindings view of the NSArrayController in the Inspector.

Drop down the contentArray property, and select “File’s Owner (NSApplication)” as the Bind to: value.

Then enter the following into the Model Key Path:


You should now see the following in the Inspector:

Binding the delegate

If you try running the application now, you’ll notice we are not quite there yet. Our controller knows about the data, our table knows about the array that populates it, but the columns don’t know what to show.

Double click on the title area of the first column in the NSTableView that is on the Window. This selects the actual table column, and also allows you to enter in a heading. Call the first one Name.

Then in the Bindings view in the Inspector, drop down the value property and select name from the Model Key Path: combo box. You should notice that our key names are pre-populated in this combo box, thanks to the binding to the controller.

Repeat the process for the second column, binding that to the date key.

Our application can now be tested, simply by running it from the command line:

$ open dist/

Try clicking on a column title and your list should automatically sort, both by name and by date. All without writing any sorting code, or any event code for the click to select columns.

Completed application

To build a distributable version of the application, re-build without the symbolic link option:

$ python py2app% open dist/

This will create an application bundle that can be deployed to a Mac OS X machine that doesn’t include PyObjC. This means that your user has no way of knowing that you’ve built a Cocoa app without writing a line of Objective C code.

Where to from here? It is worth looking through the examples included with PyObjC as they cover a wide variety of programming topics in Cocoa. In the next article, we’ll look at adding some actual features so it deserves to be called an application.

The source code for the sample application from this tutorial is available here.

Additional articles

Part 2 - Integrating Address Book

Part 3 - Making it look like an application

Bonus link

Birthday Notes - finished, open source app, based on these articles