Dynamically determine base classes on instantiation

[MRAB]

On 15/08/2012 22:17, Thomas Bach wrote:
> Hi list,
>
> I'm confronted with a strang problem I cannot find a clean solution
> for. To me it seems like I need meta-classes. Anyway, I stucked a bit
> deeper in that topic and couldn't find a proper solution neither. But,
> judge for yourselve.
>
> I want a class that determines on instantiating its base classes
> dynamically. Consider the following two use cases
>
> a = Foo(['a', 'list'])  # returns an instance that behaves like a list
> assert len(a) == 2
> assert a[0] == 'a'
> assert a == ['a', 'list']
> assert isinstance(a, list) # This would be nice, but no must-have
>
> b = Foo({'blah': 8}) # returns an instance that behaves like a dict
> assert b['blah'] == 'blah'
> assert b == {'blah': 8}
> assert isinstance(b, dict) # again, no must-have
>
> a.do_something()   # common function to both instances as defined
> b.do_something()   # in the Foo class
>
>
> What I'm currently doing something like the following:
>
> class Foo(object):
>
>      def __init__(self, obj):
>          self._obj = obj
>
>      def __len__(self):
>          return len(self._obj)
>
>      def __getitem__(self, name):
>          return self._obj[name]
>
>      # …
>
>      def do_something(self):
>          # do something on self._obj
>          pass
>
> Which seems ugly. Is there a way to provide the functions of `list'
> and `dict' in Foo's look-up path without having to write all the
> stubs myself?
>
Does Foo have to be a class? Couldn't it just be a factory function?