Python Macros

Jeff Shannon jeff at
Tue Oct 5 21:23:17 CEST 2004

Arich Chanachai wrote:

> What if foo doesn't know about bar(), can I tell foo that perhaps it 
> should send the bar() message

In addition to redefining __getattr__()/__setattr__(), as has already 
been suggested, it's even easier to have your calling code (which 
probably knows more about how to recover from failure than the called 
code) decide what to do.

except AttributeError:

I'd think that, in most cases, if foo doesn't understand bar(), then it 
won't know *why* you're calling bar(), and thus won't be in the best 
position to decide what other objects to try bar() on. 

To go back to the dog.fetch(cat) example -- if the dog doesn't 
understand how to fetch a cat, it probably doesn't know whether a human 
would understand how to fetch a cat, either.  On the other hand, if 
*you* are asking dog to fetch a cat, and the dog doesn't understand, 
then you'd be in a good position to decide that the dog's owner might 
understand... or that the handy Cat-Fetching Robot (tm) might be a 
better choice.  The dog can only say "I don't understand, and things I 
don't understand go to my owner," while the calling code has a good 
chance of having a meaningful fallback plan for each possible situation.

(This sort of setup, with the caller taking responsibility for failure 
rather than the callee attempting to make sure that *someone* handles 
failure, is far more common in Python than the sort of scheme that 
you're asking for.)

Jeff Shannon
Credit International

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