Deciding inheritance at instantiation?

[alex23]

On Aug 4, 6:48 am, Tobiah <t... at tobiah.org> wrote:
> I have a bunch of classes from another library (the html helpers
> from web2py).  There are certain methods that I'd like to add to
> every one of them.  So I'd like to put those methods in a class,
> and pass the parent at the time of instantiation.  Web2py has
> a FORM class for instance.  I'd like to go:
>
>         my_element = html_factory(FORM)
>
> Then my_element would be an instance of my class, and also
> a child of FORM.

I've lately begun to prefer composition over inheritance for
situations like this:

    class MyElementFormAdapter(object):
        def __init__(self, form):
            self.form = form

        def render_form(self):
            self.form.render()

    my_element = MyElementFormAdapter(FORM)
    my_element.render_form()
    my_element.form.method_on_form()

Advantages include being more simple and obvious than multiple
inheritance, and avoiding namespace clashes:

    class A(object):
        def foo(self):
            print 'a'

    class B(object):
        def foo(self):
            print 'b'

    class InheritFromAB(A, B):
        pass

    class AdaptAB(object):
        def __init__(self, a, b):
            self.a = a
            self.b = b

    >>> inherit = InheritFromAB()
    >>> inherit.foo()
    a
    >>> adapt = AdaptAB(A(), B())
    >>> adapt.a.foo()
    a
    >>> adapt.b.foo()
    b