Dynamically determine base classes on instantiation

Thu Aug 16 13:03:51 EDT 2012

class Foo(object):
 def __new__(cls, arg):
 if isinstance(arg, list):
 cls = FooList
 elif isinstance(arg, dict):
 cls = FooDict
 return object.__new__(cls, arg)
class FooList(Foo, list):
 pass
class FooDict(Foo, dict):
 pass
You could even have __new__ make these Foo* classes dynamically when it encounters a new type of argument.
Chard.
On Thursday, 16 August 2012 18:54:12 UTC+2, Thomas Bach wrote:
> On Thu, Aug 16, 2012 at 05:10:43PM +0200, Hans Mulder wrote:
>> > On 16/08/12 14:52:30, Thomas Bach wrote:
>> > > 
>> > > So, my question (as far as I can see it, please correct me if I am
>> > > wrong) is less of the "How do I achieve this?"-kind, but more of the
>> > > "What is a clean design for this?"-kind. My intuitive thought was that
>> > > the `merge' function should be a part of the object returned from `F'.
>> > 
>> > The misunderstanding is that you feel F should return an object with
>> > a 'merge' method and a varying abse type, while Steven and others
>> > think that F should be a function.
>>>> OK, then my design wasn't so bad in the first place. :)
>>>> I made a class `Model' which wraps the actual type and realized
>> `merge' and `F' (with a better name, though) as classmethods of
>> `Model' in order to tie together the stuff that belongs together. By
>> the way, another need I saw for this design was that
>>>> setattr(Model(), 'foo', {'bar': int})
>>>> works, whereas 
>>>> setattr(dict(), 'foo', {'bar': int}) 
>>>> raises an AttributeError (on Python 3.2). Could someone give me the
>> buzz word (or even an explanation) on why that is so?
>>>> Thomas Bach