model.py

class User:
    name = None


class Book:
    title = None
    owner = None
    client = None


user1 = User()
user1.name = 'maio'

user2 = User()
user2.name = 'Idaho'

book1 = Book()
book1.title = 'Flotila'
book1.owner = user2
book1.purchaser = user1

python shell

[maio@maio-vs ~(master)] $ python
Python 2.5.2 (r252:60911, Mar  9 2008, 03:52:30)
[GCC 4.3.0 20080301 (prerelease) [gcc-4_3-branch revision 132801]] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class User:
...     name = None
...
>>>
>>> class Book:
...     title = None
...     owner = None
...     client = None
...
>>>
>>> user1 = User()
>>> user1.name = 'maio'
>>>
>>> user2 = User()
>>> user2.name = 'Idaho'
>>>
>>> book1 = Book()
>>> book1.title = 'Flotila'
>>> book1.owner = user2
>>> book1.purchaser = user1
>>>
>>>
>>> book1.title
'Flotila'
>>> book1.title = 'XXX'
>>> book1.title
'XXX'
>>> book1.owner
<__main__.User instance at 0x40219dac>
>>> book1.owner.name
'Idaho'
>>> book1.purchaser.name
'maio'
>>>