Using Open Scripting Extension from Python

OSA support in Python is still not 100% complete, but there is already enough in place to allow you to do some nifty things to other programs from your python program.

Actually, when we say "AppleScript" in this document we actually mean "the Open Scripting Architecture", there is nothing AppleScript-specific in the Python implementation.

In this example, we will look at a scriptable application, extract its "AppleScript Dictionary" and generate a Python interface module from that and use that module to control the application. Because we want to concentrate on the OSA details we don't bother with a real user-interface for our application.

The application we are going to script is Disk Copy, Apple's standard utility for making copies of floppies, creating files that are mountable as disk images, etc.

Python OSA architecture

Open Scripting suites and inheritance can be modelled rather nicely with with Python packages, so for each application we want to script we generate a package. Each suite defined in the application becomes a module in the package, and the package main module imports everything from all the submodules and glues all the classes (Python terminology, OSA terminology is events, AppleScript terminology is verbs) together.

A suite in an OSA application can extend the functionality of a standard suite, and this is implemented in Python by importing everything from the module that implements the standard suite and overriding anything that has been extended. The standard suites live in the StdSuite package.

This all sounds complicated, and you can do strange and wondrous things with it once you fully understand it, but the good news is that simple scripting is actually pretty simple.

Creating the Python interface module

There is a tool in the standard distribution that looks through a file for an 'AETE' or 'AEUT' resource, the internal representation of the AppleScript dictionary. This tool is called gensuitemodule.py, and lives in Mac:scripts. When we start it, it asks us for an input file and we point it to the Disk Copy executable.

Next it wants a folder where it will store the package it is going to generate. Note that this is the package folder, not the parent folder, so we navigate to Python:Mac:Demo:applescript, create a folder Disk_Copy and select that.

Next it wants the folder from which it should import the standard suites. Here you always select Python:Mac:Lib:lib-scriptpackages. (There is one exception to this rule: when you are generating StdSuites itself you select cancel, for obvious reasons).

It starts parsing the AETE resource, and for each AppleEvent suite it finds it prompts us for the filename of the resulting python module. Remember to change folders for the first module, you don't want to clutter up the Eudora folder with your python interfaces. If you want to skip a suite you press cancel and the process continues with the next suite.

Gensuitemodule may ask you questions like "Where is enum 'xyz ' declared?". This is either due to a misunderstanding on my part or (rather too common) bugs in the AETE resources. Pressing cancel is usually the right option, it will cause the specific enum not to be treated as an enum but as a "normal" type. As things like fsspecs and TEXT strings clearly are not enumerators this is correct. If someone understands what is really going on here please let me know.

Time for a sidebar. If you want to re-create the StdSuite modules you should look in one of two places. On older systems you will find the AEUT resources in System Folder:Extensions:Scripting Additions:Dialects:English Dialect. On newer systems you will find them in System Folder:Extensions:Applescript.

Let's glance at the Disk_Copy package just created. You may want to open Script Editor alongside, and have a look at how it interprets the dictionary. The main package module is in __init__.py and the only interesting bit is the Disk_Copy class, which includes the event handling classes from the individual suites. It also inherits aetools.TalkTo, which is a base class that handles all details on how to start the program and talk to it, and a class variable _signature which is the default application this class will talk to (you can override this in various when you instantiate your class, see aetools.py for details).

Let us do another sidebar. Since MacPython 2.0 this new structure, with packages per application and submodules per suite, is used. Older MacPythons had a single level of modules, with uncertain semantics. With the new structure it is possible for programs to override standard suites, as programs often do. It is a good idea to convert your own old programs to the new scheme, but if you really want the old standard suites are still available in :Mac:Lib:lib-scripting.
The Special_Events module is a nice example of a suite module. The Special_Events_Events class is the bulk of the code generated. For each verb it contains a method. Each method knows what arguments the verb expects, and it makes handy use of keyword arguments to present a palatable interface to the python programmer. You will see that each method calls some routines from aetools, an auxiliary module living in Lib:toolbox which contains some other nifty AppleEvent tools as well. Have a look at it sometime, there is (of course) no documentation yet.

The other thing you notice is that each method calls self.send, this comes from the aetools.TalkTo baseclass.

After the big class we get a number of little class declarations. These declarations are for the (appleevent) classes and properties in the suite. They allow you to create object IDs, which can then be passed to the verbs. For instance, to get the name of the sender of the first message in mailbox inbox you would use mailbox("inbox").message(1).sender. It is also possible to specify this as sender(message(1, mailbox("inbox"))), which is sometimes needed because these classes don't always inherit correctly from baseclasses, so you may have to use a class or property from another suite.

There are also some older object specifiers for standard objects in aetools. You use these in the form aetools.Word(10, aetools.Document(1)) where the corresponding AppleScript terminology would be word 10 of the first document. Examine the two modules mentioned above along with the comments at the end of your suite module if you need to create more than the standard object specifiers.
Next we get the enumeration dictionaries, which allow you to pass english names as arguments to verbs, so you don't have to bother with the 4-letter type code. So, you can say 
	diskcopy.create(..., filesystem="Mac OS Standard")
as it is called in Script Editor, in stead of the cryptic lowlevel 
	diskcopy.create(..., filesystem="Fhfs")

Finally, we get the "table of contents" of the module, listing all classes and such by code, which is used by gensuitemodule.

Using a Python suite module

Now that we have created the suite module we can use it in a Python script. In older MacPython distributions this used to be a rather complicated affair, but with the package scheme and with the application signature known by the package it is very simple: you import the package and instantiate the class, as 
	talker = Disk_Copy.Disk_Copy(start=1)
You will usually specify the start=1: it will run the application if it is not already running. You may want to omit it if you want to talk to the application only if it is already running, or if the application is something like the Finder.

Looking at the sourcefile makedisk.py we see that it starts with some imports. The main program itself is a wonder of simplicity. We create the object that talks to Disk Copy, creates a disk and mounts it.

The exception handling does need a few comments, though. Since AppleScript is basically a connectionless RPC protocol nothing happens when we create to talker object. Hence, if the destination application is not running we will not notice until we send our first command. There is another thing to note about errors returned by AppleScript calls: MacOS.Error is raised for all of the errors that are known to be OSErr-type errors, server generated errors raise aetools.Error.

Scripting Additions

If you want to use any of the scripting additions (or OSAXen, in everyday speech) from a Python program you can use the same method as for applications, i.e. run gensuitemodule on the OSAX (commonly found in System Folder:Extensions:Scripting Additions or something similar). There is one minor gotcha: the application signature to use is 'MACS'.

There are two minor points to watch out for when using gensuitemodule on OSAXen: they appear all to define the class System_Object_Suite, and a lot of them have the command set in multiple dialects. You have to watch out for name conflicts, so, and make sure you select a reasonable dialect (some of the non-english dialects cause gensuitemodule to generate incorrect Python code).

Further Reading

If you want to look at more involved examples of applescripting look at the standard modules findertools and nsremote, or (possibly better, as it is more involved) fullbuild from the Mac:scripts folder.