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Sender: "John O'Hagan" <johnmohagan@gmail.com>
Date: Mon, 22 Aug 2011 23:08:50 +1000
From: John O'Hagan <research@johnohagan.com>
To: python-list@python.org
Subject: Re: Adding modified methods from another class without subclassing
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On Mon, 22 Aug 2011 15:27:36 +1000
Steven D'Aprano <steve+comp.lang.python@pearwood.info> wrote:

> On Mon, 22 Aug 2011 03:04 pm John O'Hagan wrote:
> 
> > The "pitches" attribute represents the instances and as such I found
> > myself adding a lot of methods like:
> > 
> > def __getitem__(self, index):
> >     return self.pitches[index]
> > 
> > def __len__(self):
> >     return len(self.pitches)
> 
> 
> Looks like a call for (semi-)automatic delegation!
> 
> Try something like this:
> 
> 
> # Untested
> class MySeq(object):
>     methods_to_delegate = ('__getitem__', '__len__', ...)
>     pitches = ...  # make sure pitches is defined
>     def __getattr__(self, name):
>         if name in self.__class__.methods_to_delegate:
>             return getattr(self.pitches, name)
>         return super(MySeq, object).__getattr__(self, name)
>         # will likely raise AttributeError

Thanks, this looks promising. I didn't know about __getattr__ or delegation. This example doesn't seem to work as is for special methods beginning with "__" (e.g.: "TypeError: object of type 'MyList' has no len()"). It seems that __getattr__ is not called for special methods. Also, it doesn't immediately suggest to me a way of modifying method calls (maybe __setattr__?). But it's certainly a neater way to get methods to operate on the attribute. I'm looking into it, and delegation generally.

However, I don't understand what the super call is doing. If the method isn't delegated, shouldn't it just fall back to getattr(self, name)?

Thanks and regards,

John