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

[Demetrius Cassidy]

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On Sat, Jul 2, 2011 at 10:50 PM, Demetrius Cassidy <dcassidy36 at gmail.com>wrote:

> On Sat, Jul 2, 2011 at 10:45 PM, Phil Thompson <
> phil at riverbankcomputing.com> wrote:
>
>> 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__()
>>
>
>   The super thing was a typo.
>
>>
>> > 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
>>
>
> So if /Abstract/ does not mean the class has pure virtual methods and
> cannot be instantiated itself, in which scenario should it be used? I'm a
> little confused here.
>
>
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