Re: [Pyobjc-dev] Default to returning (void)?

[Bob I. <bo...@re...>]

On Feb 15, 2004, at 10:38 PM, b.bum wrote:

>
> This is going to break a few things, but better to rip off the 
> band-aid now than wait....
>
> Currently, if I declare a standard IB action or set method (such as 
> the following)....
>
>     def setContent_(self, someContent):
>         print "Setting content...."
>         self._content = someContent
>
> .... I have to immediately follow the declaration with the following 
> ...
>
>     setContent_ = selector(setContent_, signature='v@:@')
>
> ... if I wait the method to be compatible with Key/Value Observation 
> or otherwise match the signature of the typical setter or IBAction.
>
> When PyObjC was originally written -- 1994 or so -- the NeXTSTEP 
> standard was for Obj-C methods to default to returning (id) -- to end 
> with 'return self;'.   The goal was to support a Lisp Like method 
> chaining of the [foo doBar] doBaz] doBob] sayFred] form.
>
> When OpenStep rolled around, the ObjC world shifted to methods 
> defaulting to not returning anything.   That is, to be declared as 
> returning (void).   This has many advantages for very little cost.   
> In particular, it means that remote invocation models--  cross thread 
> or inter-process-- can actually do one-way calls without having to 
> carry a proxy back across the wire.   It also means that apps didn't 
> explode because a developer assumes 'return self' when a method really 
> returns something else (something that happened a number of times!).
>
> Of course, with Python we have no notion of "return type".   A method 
> may or may not return anything.   Right now, PyObjC assumes a return 
> of (id).  In ObjC parlance, the assumed signature prefix is '@@:'.
>
> Is there any way that we can make it a bit more natural for the 
> developer to declare IBActions or setters in PyObjC such that the 
> resulting methods have signature 'v@:@'?

The only way I can think of is to analyze the bytecode before wrapping 
functions..

 >>> import dis
 >>> def setContent_(self, someContent):
...     print "Setting content..."
...     self._content = someContent
...
 >>> dis.dis(setContent_)
   2           0 LOAD_CONST               1 ('Setting content...')
               3 PRINT_ITEM
               4 PRINT_NEWLINE

   3           5 LOAD_FAST                1 (someContent)
               8 LOAD_FAST                0 (self)
              11 STORE_ATTR               2 (_content)
              14 LOAD_CONST               0 (None)
              17 RETURN_VALUE

At least in Python 2.3.x, the only way to generate "LOAD_CONST 0 (None) 
/ RETURN_VALUE" is either a bare return, or no return at all.  If you 
analyze the bytecode for a function, and it has no other RETURN_VALUE 
pairs.. then you can safely assume that it should default to void.

-bob