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How to use __getattribute__ to access a class attribute?

Ruud de Jong ruud.de.jong at consunet.nl
Mon Mar 1 14:58:43 EST 2004 
Shalabh Chaturvedi schreef:
> Ruud de Jong wrote:
> 
>> I have the situation where I need to construct the name
>> of a static method, and then retrieve the corresponding
>> function from a class object.
> 
> 
> If what you're trying to do is get C.a (where C is the class and a is 
> the name of the staticmethod) but all you have is the class C and a 
> string mname = "a", then just getattr(C, mname) should work. Generally 
> this works for any attribute access where you have the attribute name in 
> another variable.

Erh, hmm, rrriight... Built-in functions. <blush/>
Completely forgot about that one -- too focussed
on trying to do everything with object methods and such.
I really need to pay more attention to those built-in functions.

Well, that solves my immediate problem.
I knew there had to be a simple solution :-)

> 
> Another way to get the naked function, which also works for 
> classmethods, is to do C.__dict__['a']. See comments below that explain 
> what you see.

Well, that's not true, at least not on my system (XP, Python 2.3.3):

 >>> class C(object):
	def a():
		print "static method a"
	a = staticmethod(a)

	
 >>> C.__dict__['a']
<staticmethod object at 0x00AADD10>

 > [snipped]

> To repeat the same experiment, you should call 
> type(C).__getattribute__(C, 'a'). Note that in this case C *has* its own 
> __getattribute__ attribute (which is meant for instances of C, not C 
> itself). 

C did not have its own __getattribute__ method. It inherited it
from object.

I still find this whole thing slightly ambiguous.
A class C is itself an object, an instance of 'type',
just as x = C() leads to x being an instance of class C.
But x.__getattribute__ and C.__getattribute__ return
different objects.

> In fact you do this later, and it works. Hmm, at first glance 
> I'd expect what you do above to return an exception if passed C instead 
> of an instance of C (like any other well behaved method). I don't know 
> why that doesn't happen.

I would also have expected an exception. The strange thing, as I see
it, is that x.__getattribute__ and type(C).__getattribute__ both give 
the correct result, but that C.__getattribute__ *almost* gives the
correct result, but stops at calling the __get__ function on the
desciptor object.

Anyway, now that you've kindly reminded me of the existance of getattr,
I'll not spend any more time trying to understand this.

Thanks,

Ruud
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