[PyKDE] convertSubClass() -- I introduced a bug :-(

[Dave Cuthbert]

A big mea culpa here...

Phil, earlier we thought that the arguments to PyType_IsSubType() were 
switched for subclass converters; I emailed you directly, and you went 
ahead and made the change in the SIP tree (post 4.0rc3).

I think we made another mistake in understanding what SIP was doing.

In pseudocode form, the original code in rc3 looks like:

convertSubClass(obj_type, obj) is
   ## obj_type is possibly a superclass of obj -- we want to find out
   ## the real type of obj

   foreach module in loaded_modules do
     if module has subclass_converters then
       foreach converter in subclass_converters do
         ## attempt conversion
         if PyType_IsSubType(obj_type, converter.type) then
           converter.convert(obj)
           if conversion was successful then return


We thought that converter.convert() (scc->scc_converter() in the actual 
code) could only convert to converter.type 
(scc->scc_basetype->super.type) and would reject attempts to do 
otherwise.  By this reasoning, the arguments are reversed -- the check 
should be seeing if converter.type is a subtype of obj_type for a 
potential conversion.

But that's not the case!  What converter.convert() really does is 
convert obj to the _most derived type_ of converter.type -- the 
IsSubType() check is just making sure the converter is applicable.

Now, the real bug is if class A defines a subclass converter, no other 
class derived from A can also define a subclass converter (and expect it 
to be called consistently).  A's subclass converter has to be aware of 
the entire hierarchy.

Unfortunately, fixing this is very non-trivial (IMHO).  Fortunately, for 
the QCanvasItem problem we can work around it by editing the source.

Again, my apologies for my earlier misleading analysis.

Dave