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.

If you do not have a /Developer folder, then you do not have Xcode Tools installed. See Getting the Xcode Tools.

Getting Started

Create a work directory src. Check which Python you have installed PyObjC for, by running python and checking that import Foundation works. If it does not work it could be that you have installed PyObjC for /usr/local/bin/python but Apple's /usr/bin/python comes first in your $PATH. Make sure you use the right python whereever it says python in this tutorial.
Start Interface Builder, select Cocoa Application in the new file dialog, save this file as src/MainMenu.nib.
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 nibclassbuilder script. nibclassbuilder will parse the NIB file and create a skeleton module for you. Invoke it as follows (from the src directory):
```
$ python -c "import PyObjCScripts.nibclassbuilder" MainMenu.nib > CurrencyConverter.py
```
Depending on your installation, the nibclassbuilder script 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.py with the following contents:
```
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 -A argument 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:
- double-click dist/CurrencyConverter from 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:
```
$ ./dist/CurrencyConverter.app/Contents/MacOS/CurrencyConverter
```
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 pdb if 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 /Applications/Utilities/Console.app.

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.py in 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 nibclassbuilder again 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.

Open MainMenu.nib in Interface Builder. Select the Classes view and then select the ConverterController class. 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 ConverterController class have grown a new method invertRate_().

In the MainMenu.nib main window open the MainMenu menubar. Select the Edit menu. Make sure the Menus palette is open and selected, drag a separator to the Edit menu and then drag an Item there. Double-click the item and set the text to Invert Exchange Rate.

Make the connection by control-dragging from the new Invert Exchange Rate menu item to the ConverterController instance in the Instances tab in the MainMenu.nib main window.

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 ConverterController class and it gives an error message:
```
$ ./dist/CurrencyConverter.app/Contents/MacOS/CurrencyConverter 
2004-12-09 03:29:09.957 CurrencyConverter[4454] Could not connect the action 
invertRate: to target of class ConverterController
```
Moreover, it has disabled the Invert Exchange Rate menu command and continues, so the program works as it did before, only with one more (disabled) menu item.

Debugging

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 0.
Fix the final bug by testing for rate == 0.0 in 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 src directory:

$ rm -rf dist
$ python setup.py py2app

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.

Creating your first PyObjC application.