Can a base class know if a method has been overridden?

[Bruno Desthuilliers]

Ratko a écrit :
>> If your use case is to make sure a given ('abstract') method has been
>> overriden, the canonical solution is to raise NotImplementedError in the
>> base class's implementation
> 
> I am not really interested in forcing the subclass to implement a
> method. I am interested in knowing *whether* it did implement it or
> not.
> 
> 
>> Else, this may be possible using a custom metaclass (or possibly just
>> specializing the __new__ method), but there may be better solutions
>> (depending on what you're really trying to do)..
> 
> I have a base class EvtHandler that has methods defined to handle
> certain events. You then subclass from EvtHandler and override the
> methods for the events you want to receive. If a method has been
> overridden, the base class will automatically register for those
> events to make sure that they are even delivered to this handler
> (which is why I would need to know whether a method has been
> overridden or not). Of course, there are other ways of doing this
> which would require a bit more work from the subclass... I just
> thought this would be a neat "automatic" way of registering for
> events.
> 
> For example:
> 
> class EvtHandler:
>     def __init__(self):
>         if onKey is overridden:
>              register_for_key_events()
> 
>     def onKey(self):
>         pass
> 
> 
> class MyHandler(EvtHandler):
>     def onKey(self):
>         # do something here....
> 

Ok. The simplest solution, then, is simply to not implement the method 
in the base class, ie:

class EvtHandler:
     def __init__(self):
         if hasattr(self, 'onKey'):
              register_for_key_events()

      #def onKey(self):
      #    pass


class MyHandler(EvtHandler):
     def onKey(self):
         # do something here....


Another solution is to compare the functions wrapped by the methods:

class EvtHandler:
     def __init__(self):
         onKey = getattr(self, 'onKey')
         if onKey.im_func is EvtHandler.onKey.im_func:
              register_for_key_events()

      def onKey(self):
         pass


class MyHandler(EvtHandler):
     def onKey(self):
         # do something here....


HTH