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

[Phil Thompson]

On Wednesday 07 April 2004 9:29 pm, Dave Cuthbert wrote:
> 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.

Yes. I've reverted the change and added a big comment :)

> 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.

Can you explain more? Each converter handles a non-overlapping part of the 
class tree. They are just called until one of them recognises the object as 
something it knows the specific type of.

Phil