Skip to main content

Role based development

Project description

Library for Role based development.

Pythonic implementation of the DCI (Data Context Interaction) pattern (

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.



Using Roles

As a basic example, consider a domain class:

>>> class Person(object):
...     def __init__(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)

… and can do carpenter things:

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

See for more examples.


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):
NoRoleException: No role found for <class 'Cat'>

Project details

Download files

Download the file for your platform. If you're not sure which to choose, learn more about installing packages.

Source Distribution

roles-0.5.0.tar.gz (5.9 kB view hashes)

Uploaded source

Supported by

AWS AWS Cloud computing Datadog Datadog Monitoring Facebook / Instagram Facebook / Instagram PSF Sponsor Fastly Fastly CDN Google Google Object Storage and Download Analytics Huawei Huawei PSF Sponsor Microsoft Microsoft PSF Sponsor NVIDIA NVIDIA PSF Sponsor Pingdom Pingdom Monitoring Salesforce Salesforce PSF Sponsor Sentry Sentry Error logging StatusPage StatusPage Status page