Re: [Pyobjc-dev] Accessing python classes in obj-c

[b.bum <bb...@ma...>]

On Dec 8, 2003, at 12:17 PM, Jiva DeVoe wrote:
> So I understand that I can create Python files and the templates 
> supplied with PyObjC automatically will load those Python files and do 
> what ever I want them to.  I also understand if I instantiate a python 
> class in python, and access it in obj-c, I will get a proxy object to 
> the python object.  However, what if I have an objective-c 
> application, and I want to write a portion of it in python.  So, in 
> other words, I want to be able to create a class in Python, and 
> instantiate an object in objective-c that is of that class.  How would 
> I do that? Would I have to use something like SWIG?

No SWIG necessary.

If you subclass an Objective-C class from Python, the resulting class 
will look/feel/work just like any other Objective-C class.   To ensure 
that selector mapping-- method mapping-- works correctly, you will need 
to make sure that method's in the Python implementation are correctly 
named (i.e. -setObject:forKey: is setObject_forKey_ and not 
setObjectForKey ).

Because Python is a "lazily linked" interpreted language, you have to 
ensure that any classes you want to use have been defined before they 
are used.   This causes two issues;  runtime and compile time.

Runtime is easy -- just make sure you import the class in Python before 
it is used.

Compile time is a bit harder.   In particular, you can't refer to any 
class implemented in python directly because there won't be a real 
class object to link against as the source is compiled.   To solve this 
problem, I have used an abstract superclass implemented in Objective-C 
(a class that simply declares all the methods where the implementation 
is stubbed out) and a concrete subclass implemented in Python.  To the 
abstract superclass-- the objc class-- I have added a factory method 
that instantiates and returns the concrete subclass.

This is effectively exactly how class clusters work.

Finally, you have to trigger the execution of a bit of python code from 
somewhere in your code for all of this to work.   The easiest solution 
would be to simply grab the main() function that uses/initializes the 
embedded interpreter and use that within your app.   Then your main.py 
could import whatever classes you need to start the application and you 
could go from there.

b.bum