[PyQt] SIP: deriving a python class from a C++ ABC results in a TypeError exception

[Phil Thompson]

On Sat, 2 Jul 2011 17:39:14 +0000, Demetrius Cassidy
<dcassidy36 at gmail.com>
wrote:
> I have a C++ class which is declared with pure virtual methods, but if I
> derive from this class in python and implement the pure virtual methods,
I
> get an exception:
> TypeError: pyvoip.opal.OpalPCSSEndPoint cannot be instantiated or
> sub-classed
> 
> My python class is declared as following:
> 
> class PCSSEndPoint(OpalPCSSEndPoint):
>   def __init__(self):
>   super(OpalPCSSEndPoint, self).__init__()

That should be super(PCSSEndPoint, self).__init__()

> def OnShowIncoming(self, connection):
> return True
> 
> def OnShowOutgoing(self, connection):
> return True
> 
> def GetMediaFormats(self):
>   return []
> 
> 
> SIP definitions:
> 
> class OpalPCSSEndPoint : OpalLocalEndPoint /Abstract/
> {
> 
> /**Call back to indicate that remote is ringing.
> If false is returned the call is aborted.
> 
> The default implementation is pure.
> */
> virtual PBoolean OnShowIncoming(
> const OpalPCSSConnection & connection /NoCopy/ ///< Connection having
event
> ) = 0;
> 
> /**Call back to indicate that remote is ringing.
> If false is returned the call is aborted.
> 
> The default implementation is pure.
> */
> virtual PBoolean OnShowOutgoing(
> const OpalPCSSConnection & connection /NoCopy/ ///< Connection having
event
> ) = 0;
> 
> };
> 
> Now the base class of OpalPCSSEndPoint is also an ABC, as it derives
from
> an
> ABC but does not re-define the pure virtual function:
> 
> class OpalLocalEndPoint : OpalEndPoint /Abstract/
> {
>   ...
> };
> 
> class OpalEndPoint /Abstract/
> {
> virtual OpalMediaFormatList GetMediaFormats() const = 0;
> };
> 
> I tried removing  /Abstract/ from OpalLocalEndPoint, but it made no
> difference.  Any idea what's wrong in this scenario? The only way this
> works, is if I subclass OpalPCSSEndPoint in C++ and then in Python
create a
> class derived from my OpalPCSSEndPoint subclass.

/Abstract/ is the cause of the problem. It doesn't mean that the C++ is an
ABC, it means that the class contains other (unspecified) abstract methods.

Phil