Behaviour of classes (tired of writing too much)

[Steve Holden]

mpn at mic.dtu.dk wrote:
> How do I catch any reference to an instance of a class, i.e., I want to
> run some code as soon as an instance of a class is used in any way.
> (and I don't want to define all of __add__, __ge__ etc etc etc etc etc)
> 
> The reason for the question is that I want to simplify the Message
> Passing Interface (MPI) calls. I find it irritating that I have to
> start a non-blocking send/receive and then check that the data has
> arrived. Why not automagically check/wait for the data when it is
> needed?
> 
> The second part that I also need to make that idea work is the ability
> to change the instance to the received data, i.e., self=received data,
> and then launch the original reference.
> 
> So I guess that the code I want to write is:
> 
> class MPI_tmp:
>        # temporary object that changes ITSELF into another object when
> called in any way
>        def __on_any_call__(self):
>                self=newobject()
>                self.__run_the_anycall__()
> 
> which probably doesn't work :-)
> 
> Greatful for any ideas / Magnus
> 
Maybe it could make a damned fine cup of coffee as well ;-) ?

That's a pretty tall order, and mutating self isn't as simple as you 
think. However you *can* change an instance's __class__ attribute 
dynamically, which might be a way to get where you want. Not sure about 
the "trapping all accesses" bit, though. __getattr__() could give you 
access to undefined attributes.

For new_style classes you can implement __getattribute__() to trap *any* 
attribute access. This would be somewhat slow, but might be acceptable 
if you were then changing the instance's class to something that 
*didn't* implement __getattribute__().

regards
  Steve
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