[Pyobjc-dev] defining class methods, and, is this insane?

[Matthew A. <man...@gm...>]

Hi --

After much hunting with google and through the content on the pyobjc
website, I was unable to find documentation on how one creates a class
method.  I was, in particular, wanting to add a class method to an
existing Foundation class (for use only on the Python side of the
bridge, though I can imagine wanting to have access from the
Objective-C runtime as well).  Is there an official, approved of way
of doing this?

After some experimentation and poking at source code, I did get this to work:

>>> def foo(arg, *args):
...   print arg, args
...
>>> foo = objc.selector(foo)
>>> NSObject.__class__.foo = foo
>>> NSObject.foo()
<objective-c class NSObject at 0xa06bdcc0> ()
>>> NSObject.foo(1, 2, 3)
<objective-c class NSObject at 0xa06bdcc0> (1, 2, 3)

I'm unsure if setting attributes of a class's __class__ attribute is
frowned upon for some reason, or if there were a better way.  Note, I
haven't tried to access any such function from Objective-C code.  If
someone knows, after doing the above, would it work?

The hunt to figure this out came out of a desire to have an alternate
'syntax' to call objective-C methods from python code.  Instead of:

>>> d = NSMutableDictionary.alloc().init()
>>> d.setObject_forKey_(5, "foo")

I wanted the option of saying something more like:

>>> d = NSMutableDictionary.alloc().init()
>>> d.call( "setObject:", 5,
            "forKey:", "foo")

Mostly for aesthetic/readibility (and sane line wrapping in the editor) reasons.

I was able to write such a function "call", and with modest testing,
it seems to work just fine.  I'm wondering a few things:
* is there some facility that I missed that already allows for
something like this?
* is this a bad and/or insane idea?  if so, why?  what sort of gotchas
are lurking out there if I were to add a "call" function to NSObject
(both instance and class method) and then use it in my code?
* if this is not an insane idea, and there is no facility like this
already in place, should there / could there be something like this
available in pyobjc?

My little proof of concept code is below.

Thanks,
Matt Anderson

======================================================================
#!/usr/bin/env python

from Foundation import *
import objc

def call(self, *arg):
    if len(arg) == 0:
        return None
    elif len(arg) == 1:
        sel = arg[0]
        return getattr(self,sel)()
    elif len(arg) % 2 == 0:
        sel = "".join(arg[::2]).replace(":","_")
        callargs = arg[1::2]
        return getattr(self,sel)(*callargs)
    else:
        raise Exception("selector plus arguments must be even in number!")

call = objc.selector(call)
NSObject.call = call
NSObject.__class__.call = call


d = NSMutableDictionary.alloc().init()
d.call( "setObject:", 5,
        "forKey:", "foo")
print d
# prints {
#    foo = 5;
#}
print d.call("valueForKey:", "foo")
# prints 5
print d.call("className")
# prints NSCFDictionary
print NSUserDefaults.call('standardUserDefaults')
# prints <NSUserDefaults: 0x355870>

o = NSObject.alloc().init()
print NSObject.call("description")
# prints NSObject
print o.call("description")
# prints <NSObject: 0x361810>