initializing mutable class attributes

[Dan Perl]

I found a small hole in the initial code suggestion and I fixed it.  Try and
find the difference:
    class test(object):
        def __new__(typ, *args, **kwargs):
            obj = object.__new__(typ)
            obj.attr1 = 666
            typ.__init__(obj, *args, **kwargs)
            return obj

    class derived(test):
        def __init__(self, arg1, arg2):
            self.attr2 = arg1
            self.attr3 = arg2

    d = derived(111, 222)
    print d.attr1, d.attr2
    print isinstance(d, test)

So, what do people think about it?  Alex, would this be good for a recipe in
the Python Cookbook?

Dan

"Dan Perl" <dperl at rogers.com> wrote in message
news:CZoZc.159227$UTP.62107 at twister01.bloor.is.net.cable.rogers.com...
> After all this discussion, what do people think about this code?
>     class test(object):
>         def __new__(typ):
>             obj = object.__new__(typ)
>             obj.attr1 = 666
>             return obj
>
>     class derived(test):
>         def __init__(self):
>             self.attr2 = 111
>
>     d = derived()
>     print d.attr1, d.attr2
>     print isinstance(d, test)
>
> The output:
>     666 111
>     True
>
> At least it works.  The 'test' superclass defines an instance attribute
> attr1 and it initializes it with a default.  The 'derived' subclass
inherits
> the attribute (implicitly, BTW ;-)) and that happens without an __init__
in
> the superclass that would have to be invoked in the subclass.
>
> Dan
>
>