Program to an interface, not an implementation

[Mark McEahern]

[Egbert Bouwman]
> My next problem is on nearly the same page (17) of the GoF book:
> class versus interface (== type) inheritance, but in the python context.

As the following shows, inheritance in Python is implementation inheritance.
In this particular case, the subclass inherits a method they MUST override,
but you could easily provide an implementation that actually does something
too.  I'm not sure whether this answers your question, though--I haven't
been following this thread.

class PersistentBase:

    def save(self):
        raise NotImplementedError

class Invoice(PersistentBase):

    pass

i = Invoice()

try:
    i.save()
except NotImplementedError:
    pass
else:
    raise

Cheers,

// mark

-