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Role based development

Project description

Library for Role based development.

Pythonic implementation of the DCI (Data Context Interaction) pattern (http://www.artima.com/articles/dci_vision.html).

The big difference with mixins is that this role is applied only to the subject instance, not to the subject class (alas, a new class is constructed).

Roles can be assigned and revoked. Multiple roles can be applied to an instance. Revocation can happen in any particular order.

Homepage: http://amolenaar.github.com/roles

Releases: http://pypi.python.org/pypi/roles

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.

Factories

In most cases instances will require specific implementations of a certain role. This can be done by decorating the specific role implementations with 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'>

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