[Tutor] inheriting different builtin types

[Kent Johnson]

On Wed, Feb 4, 2009 at 8:27 AM, spir <denis.spir at free.fr> wrote:
> Hello,
>
> I have a strange problem and cannot see a clear method to solve it.
> I would like to build a custom type that is able to add some informational attributes and a bunch attribute to a main "value". The outline is then:
>
> class Custom(X):
>        def __init__(self, value, more):
>                X.__init__(self)
>                <define info attributes>
>        <custom methods>
>
> The base class is here to inherit value's behaviour and to avoid writing obj.value all the time. For this type will be heavily used by client code.
> The point is, this value may actually be of several builtin types. Logically, X is value.__class__. I imagine there are solutions using a factory, or __new__, or maybe metaclass (never done yet)? I cannot find a working method myself.

If I understand correctly, you have two problems:
- creating a class with a dynamic base type
- creating a subclass of a built-in value type such as int

The first is easily solved with a factory method to create the classes:
def make_value_class(base_):
  class Derived(base_):
    # etc
  return Derived

IntValue = make_value_class(int)

Subclassing builtin types is tricky because the value has to be
assigned in the __new__() method, not in __init__(). See for example
http://www.python.org/download/releases/2.2.3/descrintro/#__new__ or
search the tutor archives for __new__.

Note that subclasses of builtin types are not as useful as you might
hope because they are not preserved by binary operations. For example
given a working IntValue class, if you did
x = IntValue(3)
y = IntValue(5)
z = x+y
type(z) is int, not IntValue, so all the special sauce is lost.