Printing the arguments of an attribute in a class

Alf P. Steinbach alfps at start.no
Sun Feb 28 16:00:19 CET 2010


* vsoler:
> I have a class that is a wrapper:
> 
> class wrapper:
>     def __init__(self, object):
>         self.wrapped = object
>     def __getattr__(self, attrname):
>         print 'Trace: ', attrname
>         #print arguments to attrname, how?
>         return getattr(self.wrapped, attrname)
> 
> I can run it this way:
> 
>>>> x = wrapper([1,2,3])
>>>> x.append(4)
> Trace:  append
>>>> x.wrapped
> [1, 2, 3, 4]
> 
> I am able to capture the attribute name to x (that is, append).
> However, I do not know how to capture and print all of its arguments
> (in this case number 4).
> 
> How should I proceed?

If your goal is just learning then in your __getattr__ you might return a 
wrapper for the attribute instead of the attribute itself. Equip the wrapper 
with a __call__ method if it is a method. And equip it with other special 
methods as appropriate.

I can imagine that that approach will lead to some practical problems, but it 
may be great for learning.

If your goal is tracing, then I suggest looking at the "trace" module.

If your goal is something else purely practical, like intercepting method calls 
to do arbitrary things (logging, marshaling, whatever) then I suspect that it 
might getspretty complicated, hairy. For specific method calls you might just 
use subclassing, but for doing this in general, parameterized, you'd need to 
about the same kinds of things as the trace module does. So I guess then one 
idea might be to look at the source code of that module.

But if that's what you intend to do, then best check first if there is an 
existing solution. ParcPlace did this thing for a number of languages and 
introduced a special term for it, I can't recall but something like 
"cross-whatever mumbo jumbo concerns" plus one single catchy name. There might 
be an existing Python implementation.


Cheers & hth.,

- Alf



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