Type emulation issues with new style classes

Chris feb04.20.netman at spamgourmet.com
Sat Feb 28 19:08:02 CET 2004

I tried a couple variation of that, and __getattr__, when defined in a
metaclass is never called when accessing an attribute on an instance of a
class derived from that metaclass.

Here is some testing I did:

class Zmeta(type):
    def __getattribute__(*args):
        raise Exception('called __getattribute__ with %s' % str(args))
    def __getattr__(*args):
        raise Exception('called __getattr__ with %s' % str(args))

Z = Zmeta('Z', (), {'value': 42})
z = Z()

>>> int(z)
TypeError: int() argument must be a string or a number
>>> z.__int__
AttributeError: 'Z' object has no attribute '__int__'
>>> Z.__int__
Exception: called __getattribute__ with (<class '__main__.Z'>, '__int__')

It appears (and is confirmed in: Metaclass Programming in Python Pt. 2
http://www-106.ibm.com/developerworks/library/l-pymeta2/?ca=dnt-434) that
metaclass attributes are available to instances (classes) but not instances
of instances.


"Michael Hudson" wrote:
> > Is there any way to make the class Z behave the same way as class Y?
> You need a custom metaclass for Z, and implement __getattr__ there.
> HTH.
> Cheers,
> mwh
> -- 
>   Lisp does badly because we refuse to lie.  When people ask us if
>   we can solve insoluble problems we say that we can't, and because
>   they expect us to lie to them, they find some other language
>   where the truth is less respected.   -- Tim Bradshaw, comp.lang.lisp

More information about the Python-list mailing list