[Python-3000] PEP 3133: Introducing Roles

Mon May 14 07:36:55 CEST 2007

PEP: 3133
Title: Introducing Roles
Version: $Revision$
Last-Modified: $Date$
Author: Collin Winter <collinw at gmail.com>
Status: Draft
Type: Standards Track
Requires: 3115, 3129
Content-Type: text/x-rst
Created: 1-May-2007
Python-Version: 3.0
Post-History: 13-May-2007

Abstract
========

Python's existing object model organizes objects according to their
implementation.  It is often desirable -- especially in
duck typing-based language like Python -- to organize objects by
the part they play in a larger system (their intent), rather than by
how they fulfill that part (their implementation).  This PEP
introduces the concept of roles, a mechanism for organizing
objects according to their intent rather than their implementation.

Rationale
=========

In the beginning were objects.  They allowed programmers to marry
function and state, and to increase code reusability through concepts
like polymorphism and inheritance, and lo, it was good.  There came
a time, however, when inheritance and polymorphism weren't enough.
With the invention of both dogs and trees, we were no longer able to
be content with knowing merely, "Does it understand 'bark'?"
We now needed to know what a given object thought that "bark" meant.

One solution, the one detailed here, is that of roles, a mechanism
orthogonal and complementary to the traditional class/instance system.
Whereas classes concern themselves with state and implementation, the
roles mechanism deals exclusively with the behaviours embodied in a
given class.

This system was originally called "traits" and implemented for Squeak
Smalltalk [#traits-paper]_.  It has since been adapted for use in
Perl 6 [#perl6-s12]_ where it is called "roles", and it is primarily
from there that the concept is now being interpreted for Python 3.
Python 3 will preserve the name "roles".

In a nutshell: roles tell you *what* an object does, classes tell you
*how* an object does it.

In this PEP, I will outline a system for Python 3 that will make it
possible to easily determine whether a given object's understanding
of "bark" is tree-like or dog-like.  (There might also be more
serious examples.)

A Note on Syntax
----------------

A syntax proposals in this PEP are tentative and should be
considered to be strawmen.  The necessary bits that this PEP depends
on -- namely PEP 3115's class definition syntax and PEP 3129's class
decorators -- are still being formalized and may change.  Function
names will, of course, be subject to lengthy bikeshedding debates.

Performing Your Role
====================

Static Role Assignment
----------------------

Let's start out by defining ``Tree`` and ``Dog`` classes ::

  class Tree(Vegetable):

    def bark(self):
      return self.is_rough()

  class Dog(Animal):

    def bark(self):
      return self.goes_ruff()

While both implement a ``bark()`` method with the same signature,
they do wildly different things.  We need some way of differentiating
what we're expecting. Relying on inheritance and a simple
``isinstance()`` test will limit code reuse and/or force any dog-like
classes to inherit from ``Dog``, whether or not that makes sense.
Let's see if roles can help. ::

  @perform_role(Doglike)
  class Dog(Animal):
    ...

  @perform_role(Treelike)
  class Tree(Vegetable):
    ...

  @perform_role(SitThere)
  class Rock(Mineral):
    ...

We use class decorators from PEP 3129 to associate a particular role
or roles with a class.  Client code can now verify that an incoming
object performs the ``Doglike`` role, allowing it to handle ``Wolf``,
``LaughingHyena`` and ``Aibo`` [#aibo]_ instances, too.

Roles can be composed via normal inheritance: ::

  @perform_role(Guard, MummysLittleDarling)
  class GermanShepherd(Dog):

    def guard(self, the_precious):
      while True:
        if intruder_near(the_precious):
          self.growl()

    def get_petted(self):
      self.swallow_pride()

Here, ``GermanShepherd`` instances perform three roles: ``Guard`` and
``MummysLittleDarling`` are applied directly, whereas ``Doglike``
is inherited from ``Dog``.

Assigning Roles at Runtime
--------------------------

Roles can be assigned at runtime, too, by unpacking the syntactic
sugar provided by decorators.

Say we import a ``Robot`` class from another module, and since we
know that ``Robot`` already implements our ``Guard`` interface,
we'd like it to play nicely with guard-related code, too. ::

  >>> perform(Guard)(Robot)

This takes effect immediately and impacts all instances of ``Robot``.

Asking Questions About Roles
----------------------------

Just because we've told our robot army that they're guards, we'd
like to check in on them occasionally and make sure they're still at
their task. ::

  >>> performs(our_robot, Guard)
  True

What about that one robot over there? ::

  >>> performs(that_robot_over_there, Guard)
  True

The ``performs()`` function is used to ask if a given object
fulfills a given role.  It cannot be used, however, to ask a
class if its instances fulfill a role: ::

  >>> performs(Robot, Guard)
  False

This is because the ``Robot`` class is not interchangeable
with a ``Robot`` instance.

Defining New Roles
==================

Empty Roles
-----------

Roles are defined like a normal class, but use the ``Role``
metaclass. ::

  class Doglike(metaclass=Role):
    ...

Metaclasses are used to indicate that ``Doglike`` is a ``Role`` in
the same way 5 is an ``int`` and ``tuple`` is a ``type``.

Composing Roles via Inheritance
-------------------------------

Roles may inherit from other roles; this has the effect of composing
them.  Here, instances of ``Dog`` will perform both the
``Doglike`` and ``FourLegs`` roles. ::

  class FourLegs(metaclass=Role):
    pass

  class Doglike(FourLegs, Carnivor):
    pass

  @perform_role(Doglike)
  class Dog(Mammal):
    pass

Requiring Concrete Methods
--------------------------

So far we've only defined empty roles -- not very useful things.
Let's now require that all classes that claim to fulfill the
``Doglike`` role define a ``bark()`` method: ::

  class Doglike(FourLegs):

    def bark(self):
      pass

No decorators are required to flag the method as "abstract", and the
method will never be called, meaning whatever code it contains (if any)
is irrelevant.  Roles provide *only* abstract methods; concrete
default implementations are left to other, better-suited mechanisms
like mixins.

Once you have defined a role, and a class has claimed to perform that
role, it is essential that that claim be verified.  Here, the
programmer has misspelled one of the methods required by the role. ::

  @perform_role(FourLegs)
  class Horse(Mammal):

    def run_like_teh_wind(self)
      ...

This will cause the role system to raise an exception, complaining
that you're missing a ``run_like_the_wind()`` method.  The role
system carries out these checks as soon as a class is flagged as
performing a given role.

Concrete methods are required to match exactly the signature demanded
by the role.  Here, we've attempted to fulfill our role by defining a
concrete version of ``bark()``, but we've missed the mark a bit. ::

  @perform_role(Doglike)
  class Coyote(Mammal):

    def bark(self, target=moon):
      pass

This method's signature doesn't match exactly with what the
``Doglike`` role was expecting, so the role system will throw a bit
of a tantrum.

Mechanism
=========

The following are strawman proposals for how roles might be expressed
in Python.  The examples here are phrased in a way that the roles
mechanism may be implemented without changing the Python interpreter.
(Examples adapted from an article on Perl 6 roles by Curtis Poe
[#roles-examples]_.)

1. Static class role assignment ::

     @perform_role(Thieving)
     class Elf(Character):
       ...

   ``perform_role()`` accepts multiple arguments, such that this is
   also legal: ::

     @perform_role(Thieving, Spying, Archer)
     class Elf(Character):
       ...

   The ``Elf`` class now performs both the ``Thieving``, ``Spying``,
   and ``Archer`` roles.

2. Querying instances ::

     if performs(my_elf, Thieving):
       ...

   The second argument to ``performs()`` may also be anything with a
   ``__contains__()`` method, meaning the following is legal: ::

     if performs(my_elf, set([Thieving, Spying, BoyScout])):
       ...

   Like ``isinstance()``, the object needs only to perform a single
   role out of the set in order for the expression to be true.

Relationship to Abstract Base Classes
=====================================

Early drafts of this PEP [#proposal]_ envisioned roles as competing
with the abstract base classes proposed in PEP 3119.  After further
discussion and deliberation, a compromise and a delegation of
responsibilities and use-cases has been worked out as follows:

* Roles provide a way of indicating a object's semantics and abstract
  capabilities.  A role may define abstract methods, but only as a
  way of delineating an interface through which a particular set of
  semantics are accessed.  An ``Ordering`` role might require that
  some set of ordering operators  be defined. ::

    class Ordering(metaclass=Role):
      def __ge__(self, other):
        pass

      def __le__(self, other):
        pass

      def __ne__(self, other):
        pass

      # ...and so on

  In this way, we're able to indicate an object's role or function
  within a larger system without constraining or concerning ourselves
  with a particular implementation.

* Abstract base classes, by contrast, are a way of reusing common,
  discrete units of implementation.  For example, one might define an
  ``OrderingMixin`` that implements several ordering operators in
  terms of other operators. ::

    class OrderingMixin:
      def __ge__(self, other):
        return self > other or self == other

      def __le__(self, other):
        return self < other or self == other

      def __ne__(self, other):
        return not self == other

      # ...and so on

  Using this abstract base class - more properly, a concrete
  mixin - allows a programmer to define a limited set of operators
  and let the mixin in effect "derive" the others.

By combining these two orthogonal systems, we're able to both
a) provide functionality, and b) alert consumer systems to the
presence and availability of this functionality.  For example,
since the ``OrderingMixin`` class above satisfies the interface
and semantics expressed in the ``Ordering`` role, we say the mixin
performs the role: ::

  @perform_role(Ordering)
  class OrderingMixin:
    def __ge__(self, other):
      return self > other or self == other

    def __le__(self, other):
      return self < other or self == other

    def __ne__(self, other):
      return not self == other

    # ...and so on

Now, any class that uses the mixin will automatically -- that is,
without further programmer effort -- be tagged as performing the
``Ordering`` role.

The separation of concerns into two distinct, orthogonal systems
is desirable because it allows us to use each one separately.
Take, for example, a third-party package providing a
``RecursiveHash`` role that indicates a container takes its
contents into account when determining its hash value.  Since
Python's built-in ``tuple`` and ``frozenset`` classes follow this
semantic, the ``RecursiveHash`` role can be applied to them. ::

  >>> perform_role(RecursiveHash)(tuple)
  >>> perform_role(RecursiveHash)(frozenset)

Now, any code that consumes ``RecursiveHash`` objects will now be
able to consume tuples and frozensets.

Open Issues
===========

Allowing Instances to Perform Different Roles Than Their Class
--------------------------------------------------------------

Perl 6 allows instances to perform different roles than their class.
These changes are local to the single instance and do not affect
other instances of the class.  For example: ::

  my_elf = Elf()
  my_elf.goes_on_quest()
  my_elf.becomes_evil()
  now_performs(my_elf, Thieving) # Only this one elf is a thief
  my_elf.steals(["purses", "candy", "kisses"])

In Perl 6, this is done by creating an anonymous class that
inherits from the instance's original parent and performs the
additional role(s).  This is possible in Python 3, though whether it
is desirable is still is another matter.

Inclusion of this feature would, of course, make it much easier to
express the works of Charles Dickens in Python: ::

  >>> from literature import role, BildungsRoman
  >>> from dickens import Urchin, Gentleman
  >>>
  >>> with BildungsRoman() as OliverTwist:
  ...   mr_brownlow = Gentleman()
  ...   oliver, artful_dodger = Urchin(), Urchin()
  ...   now_performs(artful_dodger, [role.Thief, role.Scoundrel])
  ...
  ...   oliver.has_adventures_with(ArtfulDodger)
  ...   mr_brownlow.adopt_orphan(oliver)
  ...   now_performs(oliver, role.RichWard)

Requiring Attributes
--------------------

Neal Norwitz has requested the ability to make assertions about
the presence of attributes using the same mechanism used to require
methods.  Since roles take effect at class definition-time, and
since the vast majority of attributes are defined at runtime by a
class's ``__init__()`` method, there doesn't seem to be a good way
to check for attributes at the same time as methods.

It may still be desirable to include non-enforced attributes in the
role definition, if only for documentation purposes.

Roles of Roles
--------------

Under the proposed semantics, it is possible for roles to
have roles of their own. ::

  @perform_role(Y)
  class X(metaclass=Role):
    ...

While this is possible, it is meaningless, since roles
are generally not instantiated.  There has been some
off-line discussion about giving meaning to this expression, but so
far no good ideas have emerged.

class_performs()
----------------

It is currently not possible to ask a class if its instances perform
a given role.  It may be desirable to provide an analogue to
``performs()`` such that ::

  >>> isinstance(my_dwarf, Dwarf)
  True
  >>> performs(my_dwarf, Surly)
  True
  >>> performs(Dwarf, Surly)
  False
  >>> class_performs(Dwarf, Surly)
  True

Prettier Dynamic Role Assignment
--------------------------------

An early draft of this PEP included a separate mechanism for
dynamically assigning a role to a class.  This was spelled ::

  >>> now_perform(Dwarf, GoldMiner)

This same functionality already exists by unpacking the syntactic
sugar provided by decorators: ::

  >>> perform_role(GoldMiner)(Dwarf)

At issue is whether dynamic role assignment is sufficiently important
to warrant a dedicated spelling.

Syntax Support
--------------

Though the phrasings laid out in this PEP are designed so that the
roles system could be shipped as a stand-alone package, it may be
desirable to add special syntax for defining, assigning and
querying roles.  One example might be a role keyword, which would
translate ::

  class MyRole(metaclass=Role):
    ...

into ::

  role MyRole:
    ...

Assigning a role could take advantage of the class definition
arguments proposed in PEP 3115: ::

  class MyClass(performs=MyRole):
    ...

Implementation
==============

A reference implementation is forthcoming.

Acknowledgements
================

Thanks to Jeffery Yasskin, Talin and Guido van Rossum for several
hours of in-person discussion to iron out the differences, overlap
and finer points of roles and abstract base classes.

References
==========

.. [#aibo]
   http://en.wikipedia.org/wiki/AIBO

.. [#roles-examples]
   http://www.perlmonks.org/?node_id=384858

.. [#perl6-s12]
   http://dev.perl.org/perl6/doc/design/syn/S12.html

.. [#traits-paper]
   http://www.iam.unibe.ch/~scg/Archive/Papers/Scha03aTraits.pdf

.. [#proposal]
   http://mail.python.org/pipermail/python-3000/2007-April/007026.html

Copyright
=========

This document has been placed in the public domain.

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