Implement abstract objects

The idea is to implement abstract container managed objects, much like Java Spring supports it, so that, for instance, the syntax below would
be supported.

And the purpose of using such objects would be to keep the common configuration in the parent objects which would be subclassed and customized in the child ones.

Here's the mailing list thread - http://lists.springsource.com/archives/springpython-users/2009-November/000155.html

<object id="BaseServer" class="my.Server" abstract="true">
<int id="admin-port">17200</int>
<int id="emergency-port">27200</int>
</object>

<object id="DevServer" parent="BaseServer">
<int id="http-port">8199</int>
</object>

<object id="TestServer" parent="BaseServer">
<int id="http-port">8299</int>
</object>

...

object: BaseServer
class: my.Server
properties:

• object: admin-port
  int: 17200
• object: emergency-port
  int: 27200

object: DevServer
parent: BaseServer
properties:

• object: http-port
  int: 8199

object: TestServer
parent: BaseServer
properties:

• object: http-port
  int: 8299

...

class Config(PythonConfig):

@Object
@abstract
def base_server(self):
server = my.Server()
server.admin_port = 17200
server.emergency_port = 27200

return server

@Object
@parent("base_server")
def dev_server(self, server=None):

1. 'server' would be automatically provided by the 'parent' decorator
   server.http_port = 8199
   return server

@Object
@parent("base_server")
def test_server(self, server=None):
server.http_port = 8299
return server

...