object inheritance

[Duncan Booth]

Anand <anandology at gmail.com> wrote:

> On Oct 26, 5:31 pm, "Pradeep Jindal" <praddyjin... at gmail.com> wrote:
>> Can you tell any specific use case for doing this?
> 
> I have many implementaions of a db interface.
> 
> SimpleDB - simple implementation
> BetterDB - optimized implementation
> CachedDB - an implementation with caching of queries
> RestrictedDB - implementation with permissions
> 
> Now, I want to combine these implementations and use.
> Typical use case scenarios are:
> 
> db = RestrictedDB(CachedDB(SimpleDB()))
> db = RestrictedDB(SimpleDB())
> db = RestrictedDB(BetterDB())
> db = RestrictedDB(CachedDB(BetterDB())
> db = CachedDB(SimpleDB())
> etc..
> 
> 
No, that is an argument for multiple-inheritance, mixin classes etc. You 
know when constructing the object what behaviour you want it to have. It 
isn't an argument for changing the behaviour of an existing object 
dynamically.

for an example where it is useful to extend the behaviour of objects have a 
look at Zope3 interfaces and adaptors: an object can be defined to 
implement one or more interfaces, and you can associate additional 
interfaces with objects at runtime. Sometimes an object will implement an 
interface directly, but often the implementation is taken out into a 
separate 'adaptor' class. Whenever you want to use a specified interface on 
an object you simply use the interface class as a constructor on the 
object, and that might find the appropriate adapter or it might just return 
the original object if the interface is provided directly.

e.g. see http://plone.org/documentation/tutorial/five-zope3-walkthrough

Most of the interface/adapter/object relationships in Zope are static, but 
there are interfaces which can be added/removed from objects at runtime: 
each web accessible object has a page where you can add or remove 
interfaces. For example there is an INavigationRoot interface that you can 
add to an object which makes it become the top of the navigation tree for 
that part of the site or IPhotoAlbumAble which lets an object behave as a 
photo album.

Note that this is very different than the forwarding mechanism which 
started this thread: when you use an adaptor you get something which 
implements just the specified interface, you don't have to worry about 
conflicting names for methods or attributes. Also you can have a common 
interface associated with objects which have completely different adapters 
implementing that interface.