roles 0.7.1

Using Roles

As a basic example, consider a domain class:

>>> class Person(object):
...     def __init__(self, name):
...         self.name = name
>>> person = Person("John")

The instance should participate in a collaboration in which it fulfills a particular role:

>>> from roles import RoleType
>>> class Carpenter(object):
...     __metaclass__ = RoleType
...     def chop(self):
...          return "chop, chop"

Assign the role to the person:

>>> Carpenter(person)                           # doctest: +ELLIPSIS
<Person+Carpenter object at 0x...>
>>> person                                      # doctest: +ELLIPSIS
<Person+Carpenter object at 0x...>

The person is still a Person:

>>> isinstance(person, Person)
True

… and can do carpenter things:

>>> person.chop()
'chop, chop'

See roles.py for more examples.

Context

Roles make a lot of sense when used in a context. A classic example is the money transfer example. Here two accounts are used and an amount of money is transfered from one account to the other. So, one account playes the role of source account and the other plays the role of target account.

An example can be found in example.py and example2.py.

Factories

There are cases where roles can not be implemented in a generic fashion (e.g. when dealing with legacy code). To overcome this specific role implementations can be made and assigned to specific types (classes) using the assignto() decorator.

>>> from roles import assignto
>>> @assignto(Person)
... class Biker(object):
...     __metaclass__ = RoleType
...     def bike(self):
...         return 'cycle, cycle'

>>> Biker(person)                               # doctest: +ELLIPSIS
<Person+Carpenter+Biker object at 0x...>

Assigning to a different class instance doesn’t work:

>>> class Cat(object):
...     pass
>>> Biker(Cat())                                # doctest: +ELLIPSIS
Traceback (most recent call last):
  ...
NoRoleError: No role found for <class 'Cat'>