Creating your first PyObjC application.
This document is old and hasn’t been updated for modern versions of PyObjC and Apple’s developer tools.
In this tutorial you will learn how to create your first Python Cocoa application: a simple dialog that allows you to convert amounts of money from one currency to another. Definitely easier to do with a calculator, but in the process of following the tutorial you will learn which bits of Apple’s Cocoa documentation apply to PyObjC and which bits are different, and how to adapt the different bits to PyObjC from Objective-C.
To follow the tutorial you need:
- PyObjC 1.3.1
- py2app 0.2 or later (included in the binary installer for PyObjC)
- Python 2.3 or later (note: PyObjC is NOT compatible with MacPython-OS9)
- Mac OS X 10.2 or later
- Xcode Tools (was Developer Tools for Mac OS X 10.2)
If you do not have a
/Developer folder, then you do not have Xcode Tools
installed. See Apple’s developer website <https://developer.apple.com/xcode/>
for more information on getting Xcode.
Before you start, download the
reference source package for this tutorial.
- Create a work directory
src. Check which Python you have installed PyObjC for, by running
pythonand checking that
import Foundationworks. If it does not work it could be that you have installed PyObjC for
/usr/bin/pythoncomes first in your
$PATH. Make sure you use the right python wherever it says
pythonin this tutorial.
- Start Interface Builder, select Cocoa Application
in the new file dialog, save this file as
- Proceed with the instructions as lined out in Apple’s Developing Cocoa Objective-C Applications: a Tutorial, chapter 3, just after the section “Creating the Currency Converter Interface”. Work through “Defining the Classes of Currency Converter”, “Connecting ConverterController to the Interface”, and stop at “Implementing the Classes of Currency Converter”, as we are going to do this in Python, not Objective-C. Your nib file should now be the same as step3-MainMenu.nib.
Create the skeleton Python script by running the
nibclassbuilderwill parse the NIB file and create a skeleton module for you. Invoke it as follows (from the
$ python -c "import PyObjCScripts.nibclassbuilder" MainMenu.nib > CurrencyConverter.py
Depending on your installation, the
nibclassbuilderscript may be on your
$PATH. If so, it can be invoked as such:
$ nibclassbuilder MainMenu.nib > CurrencyConverter.py
The result of this can be seen in step4-CurrencyConverter.py.
Testing the user interface
Now we need to create an build script for CurrencyConverter. To do this, create a file named
setup.pywith the following contents:
1 2 3 4 5 6 7
from distutils.core import setup import py2app setup( app=['CurrencyConverter.py'], data_files=['MainMenu.nib'], )
The result of this can be seen in step5-setup.py.
Run the setup script to create a temporary application bundle for development:
$ python setup.py py2app -A
Note that we use the
-Aargument to create an alias bundle at
dist/CurrencyConverter.app. Alias bundles contain an alias to the main script (
CurrencyConverter.py) and symlinks to the data files (
MainMenu.nib), rather than including them and their dependencies into a standalone application bundle. This allows us to keep working on the source files without having to rebuild the application. This alias bundle is similar to a ZeroLink executable for Xcode - it is for DEVELOPMENT ONLY, and will not work on other machines.
Run the program. This can be done in three ways:
dist/CurrencyConverterfrom the Finder (you won’t see the .app extension)
open it from the terminal with:
$ open dist/CurrencyConverter.app
run it directly from the Terminal, as:
The last method is typically the best to use for development: it leaves stdout and stderr connected to your terminal session so you can see what is going on if there are errors, and it allows you to interact with
pdbif you are using it to debug your application. Note that your application will likely appear in the background, so you will have to cmd-tab or click on its dock icon to see its user interface.
The other methods cause stdout and stderr to go to the Console log, which can be viewed with
When you run your script as it is now it should behave identically as when you tested your interface in Interface Builder in step 3, only now the skeleton is in Python, not Objective-C.
Writing the code
Time to actually write some code. Open
CurrencyConverter.pyin your favorite text editor. Follow Apple’s documentation again, chapter 3, section “Implementing Currency Converter’s Classes”. To translate this Objective C code to Python syntax, we will need to do some name mangling of the selectors. See An introduction to PyObjC for the details, but the short is that:
[anObject modifyArg: arg1 andAnother: arg2]
translates into the following Python code, by replacing the colons in the selector with underscores, and passing the arguments as you would with a normal Python method call:anObject.modifyArg_andAnother_(arg1, arg2)
Note that we don’t do this mangling for
Converter.convertAmount(): this method is only called by other Python code, so there is no need to go through the name mangling. Also, if we would want to make this method callable from ObjC code we may have to tell the PyObjC runtime system about the types of the arguments, so it could do the conversion. This is beyond the scope of this first tutorial, An introduction to PyObjC has a little more detail on this.
The application should now be fully functional, try it. The results of what we have up to now can be seen in step8-CurrencyConverter.py.
Extending the functionality
We are going to add one more goodie, just to show how you edit an existing application. The main problem, which may be obvious, is that we cannot run
nibclassbuilderagain because we would destroy all the code we wrote in steps 5 and 8, so we do this by hand. What we are going to do is add an “invert rate” command, because I always get this wrong: instead of typing in the exchange rate from dollars to euros I type in the rate to convert from euros to dollars.
MainMenu.nibin Interface Builder. Select the Classes view and then select the
ConverterControllerclass. In the info panel select the Attributes from the popup. Select the Actions tab, and add an action
invertRate:. You have now told Interface Builder that instances of the
ConverterControllerclass have grown a new method
MainMenu.nib mainwindow open the MainMenu menubar. Select the
Editmenu. Make sure the Menus palette is open and selected, drag a separator to the
Editmenu and then drag an
Itemthere. Double-click the item and set the text to
Invert Exchange Rate.
Make the connection by control-dragging from the new
Invert Exchange Ratemenu item to the
ConverterControllerinstance in the Instances tab in the
NOTE: you drag to the instance of
ConverterController, not to the class.
In the Info panel, Connections section, select
invertRate:and press Connect.
We know our program can’t invert rates yet, because we haven’t actually written the code to do it, but we are going to try it anyway, just to see what sort of spectacular crash we get. Alas, nothing spectacular about it: when the NIB is loaded the Cocoa runtime system tries to make the connection, notices that we have no
invertRate_()method in our
ConverterControllerclass and it gives an error message:
$ ./dist/CurrencyConverter.app/Contents/MacOS/CurrencyConverter 2004-12-09 03:29:09.957 CurrencyConverter Could not connect the action invertRate: to target of class ConverterController
Moreover, it has disabled the
Invert Exchange Ratemenu command and continues, so the program works as it did before, only with one more (disabled) menu item.
Writing the code is easy: add a method
invertRate_(self, sender)that gets the float value of
rateField, inverts it and puts it back. We deliberately forget to test for divide by zero. We run the program again, and now the menu entry is enabled. After trying it with a couple of non-zero exchange rates we try it with an exchange rate of zero (or empty, which is the same). We get a dialog box giving the Python exception, and offering the choice of continuing or quitting.
To debug this application with pdb, start the application with the following command line:
$ env USE_PDB=1 ./dist/CurrencyConverter.app/Contents/MacOS/CurrencyConverter
When running in this mode, we will get a
pdb.post_mortem(...)console in the terminal instead of the alert panel. You can see this in action if you try and invert an exchange rate of
Fix the final bug by testing for
rate == 0.0in
invertRate_(). The result is in the step12-src directory.
Creating a redistributable application
Your application is finished, and you want to run it on other computers, or
simply just move it to the
Applications folder (or anywhere else) and
insulate it from the original source code.
This can be done with the following steps from the
Now the application bundle located at
dist/CurrencyConverter.app is a fully
standalone application that should run on any computer running the same major
version of Mac OS X or later. This means that applications built on
Mac OS X 10.2 are compatible with Mac OS X 10.3, but NOT vice versa. If you
are not using an Apple-supplied version of Python, a subset of your Python
installation will be included in this application.
For more complicated examples of py2app usage to do things such as change the application’s icon, see the Examples or the py2app documentation.