python-peps/pep-3119.txt

PEP: 3119
Title: Introducing Abstract Base Classes
Version: $Revision$
Last-Modified: $Date$
Author: Guido van Rossum <guido@python.org>, Talin
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 18-Apr-2007
Post-History: Not yet posted


Abstract
========

This is a proposal to add Abstract Base Class (ABC) support to Python
3000.  It proposes:

* An "ABC support framework" which defines a metaclass, a base class,
  a decorator, and some helpers that make it easy to define ABCs.
  This will be added as a new library module named "abc".

* Specific ABCs for containers and iterators, to be added to the
  collections module.

* Specific ABCs for numbers, to be added to a new module, yet to be
  named.

* Guidelines for writing additional ABCs.


Rationale
=========

In the domain of object-oriented programming, the usage patterns for
interacting with an object can be divided into two basic categories,
which are 'invocation' and 'inspection'.

Invocation means interacting with an object by invoking its methods.
Usually this is combined with polymorphism, so that invoking a given
method may run different code depending on the type of an object.

Inspection means the ability for external code (outside of the object's
methods) to examine the type or properties of that object, and make
decisions on how to treat that object based on that information.

Both usage patterns serve the same general end, which is to be able to
support the processing of diverse and potentially novel objects in a
uniform way, but at the same time allowing processing decisions to be
customized for each different type of object.

In classical OOP theory, invocation is the preferred usage pattern, and
inspection is actively discouraged, being considered a relic of an
earlier, procedural programming style. However, in practice this view is
simply too dogmatic and inflexible, and leads to a kind of design
rigidity that is very much at odds with the dynamic nature of a language
like Python.

In particular, there is often a need to process objects in a way that
wasn't anticipated by the creator of the object class. It is not always
the best solution to build in to every object methods that satisfy the
needs of every possible user of that object. Moreover, there are many
powerful dispatch philosophies that are in direct contrast to the
classic OOP requirement of behavior being strictly encapsulated within
an object, examples being rule or pattern-match driven logic.

On the the other hand, one of the criticisms of inspection by classic
OOP theorists is the lack of formalisms and the ad hoc nature of what is
being inspected. In a language such as Python, in which almost any
aspect of an object can be reflected and directly accessed by external
code, there are many different ways to test whether an object conforms
to a particular protocol or not. For example, if asking 'is this object
a mutable sequence container?', one can look for a base class of 'list',
or one can look for a method named '__getitem__'. But note that although
these tests may seem obvious, neither of them are correct, as one
generates false negatives, and the other false positives.

The generally agreed-upon remedy is to standardize the tests, and group
them into a formal arrangement. This is most easily done by associating
with each class a set of standard testable properties, either via the
inheritance mechanism or some other means. Each test carries with it a
set of promises: it contains a promise about the general behavior of the
class, and a promise as to what other class methods will be available.

This PEP proposes a particular strategy for organizing these tests known
as Abstract Base Classes, or ABC. ABCs are simply Python classes that
are added into an object's inheritance tree to signal certain features
of that object to an external inspector. Tests are done using
isinstance(), and the presence of a particular ABC means that the test
has passed.

Like all other things in Python, these promises are in the nature of a
gentlemen's agreement - which means that the language does not attempt
to enforce that these promises are kept.


Specification
=============

The specification follows the four categories listed in the abstract:

* An "ABC support framework" which defines a metaclass, a base class,
  a decorator, and some helpers that make it easy to define ABCs.
  This will be added as a new library module named "abc".

* Specific ABCs for containers and iterators, to be added to the
  collections module.

* Specific ABCs for numbers, to be added to a new module, yet to be
  named.

* Guidelines for writing additional ABCs.


ABC Support Framework
---------------------

The abc module will define some utilities that help defining ABCs.
These are:

``@abstractmethod``
    A decorator to be used to declare abstract methods.  This should
    only be used with classes whose metaclass is (or is derived from)
    ``AbstractClass`` below.  A class containing at least one method
    declared with this decorator that hasn't been overridden yet
    cannot be instantiated.  Such a methods may be called from the
    overriding method in the subclass (using ``super`` or direct
    invocation).
    
``Abstract``
    A class implementing the constraint that it or its subclasses
    cannot be instantiated unless each abstract method has been
    overridden.  Its metaclass is ``AbstractClass``.  Note: being
    derived from ``Abstract`` does not make a class abstract; the
    abstract-ness is decided on a per-class basis, depending on
    whether all methods defined with ``@abstractmethod`` have been
    overridden.

``AbstractClass``
    The metaclass of Abstract (and all classes derived from it).  Its
    purpose is to collect the information during the class
    construction stage.  It derives from ``type``.

``AbstractInstantiationError``
    The exception raised when attempting to instantiate an abstract
    class.  It derives from ``TypeError``.

A possible implementation would add an attribute
``__abstractmethod__`` to any method declared with
``@abstractmethod``, and add the names of all such abstract methods to
a class attribute named ``__abstractmethods__``.  Then the
``Abstract.__new__()`` method would raise an exception if any abstract
methods exist on the class being instantiated.  For details see [2]_.


ABCs for Containers and Iterators
---------------------------------

The collections module will define ABCs necessary and sufficient to
work with sets, mappings, sequences, and some helper types such as
iterators and dictionary views.

The ABCs provide implementations of their abstract methods that are
technically valid but fairly useless; e.g. ``__hash__`` returns 0, and
``__iter__`` returns an empty iterator.  In general, the abstract
methods represent the behavior of an empty container of the indicated
type.

Some ABCs also provide concrete (i.e. non-abstract) methods; for
example, the ``Iterator`` class has an ``__iter__`` method returning
itself, fulfilling an important invariant of iterators (which in
Python 2 has to be implemented anew by each iterator class).

No ABCs override ``__init__``, ``__new__``, ``__str__`` or
``__repr__``.

XXX define how set, frozenset, list, tuple, dict, bytes and str derive
from these.


One Trick Ponies
''''''''''''''''

These abstract classes represent single methods like ``__iter__`` or
``__len__``.

``Hashable``
    The base class for classes defining ``__hash__``.  Its abstract
    ``__hash__`` method always returns 0, which is a valid (albeit
    inefficient) implementation.  Note: being an instance of this
    class does not imply that an object is immutable; e.g. a tuple
    containing a list as a member is not immutable; its ``__hash__``
    method raises an exception.

``Iterable``
    The base class for classes defining ``__iter__``.  Its abstract
    ``__iter__`` method returns an empty iterator.

``Iterator``
    The base class for classes defining ``__next__``.  This derives
    from ``Iterable``.  Its abstract ``__next__`` method raises
    StopIteration.  Its ``__iter__`` method returns ``self``, and is
    *not* abstract.

``Lengthy``
    The base class for classes defining ``__len__``.  Its abstract
    ``__len__`` method returns 0.  (The name is perhaps too cute; but
    the only alternatives I've come up with so far are ``Sizeable``,
    which suffers from the same affliction, and ``Finite``, which
    somehow is associated with numbers instead of sets in my mind.)

``Container``
    The base class for classes defining ``__contains__`.  Its abstract
    ``__contains__`` method returns ``False``.  Note: strictly
    speaking, there are three variants of this method's semantics.
    The first one is for sets and mappings, which is fast: O(1) or
    O(log N).  The second one is for membership checking on sequences,
    which is slow: O(N).  The third one is for subsequence checking on
    (character or byte) strings, which is also slow: O(N).  Would it
    make sense to distinguish these?  The signature of the third
    variant is different, since it takes a sequence (typically of the
    same type as the method's target) intead of an element.  For now,
    I'm using the same type for all three.


Sets
''''

These abstract classes represent various stages of "set-ness".

``Set``
    This is a finite, iterable container, i.e. a subclass of
    ``Lengthy``, ``Iterable`` and ``Container``.  Not every subset of
    those three classes is a set though!  Sets have the additional
    property (though it is not expressed in code) that each element
    occurs only once (as can be determined by iteration), and in
    addition sets implement rich comparisons defined as
    subclass/superclass tests.

    Sets with different implementations can be compared safely,
    efficiently and correctly.  Because ``Set`` derives from
    ``Lengthy``, ``__eq__`` takes a shortcut and returns ``False``
    immediately if two sets of unequal length are compared.
    Similarly, ``__le__`` returns ``False`` immediately if the first
    set has more members than the second set.  Note that set inclusion
    implements only a partial ordering; e.g. {1, 2} and {1, 3} are not
    ordered (all three of ``<``, ``==`` and ``>`` return ``False`` for
    these arguments).  Sets cannot be ordered relative to mappings or
    sequences, but they can be compared for equality (and then they
    always compare unequal).

    XXX Should we also implement the ``issubset`` and ``issuperset``
    methods found on the set type in Python 2 (which are apparently
    just aliases for ``__le__`` and ``__ge__``)?

    XXX Should this class also implement union, intersection,
    symmetric and asymmetric difference and/or the corresponding
    operators?  The problem with those (unlike the comparison
    operators) is what should be the type of the return value.  I'm
    tentatively leaving these out -- if you need them, you can test
    for a ``Set`` instance that implements e.g. ``__and__``.  Some
    alternatives: make these abstract methods (even though the
    semantics apart from the type are well-defined); or make them
    concrete methods that return a specific concrete set type; or make
    them concrete methods that assume the class constructor always
    accepts an iterable of elements; or add a new class method that
    accepts an iterable of elements and that creates a new instance.
    (I originally considered a "view" alternative, but the problem is
    that computing ``len(a&b)`` requires iterating over ``a`` or
    ``b``, and that pretty much kills the idea.)

``HashableSet``
    This is a subclass of both ``Set`` and ``Hashable``.  It
    implements a concrete ``__hash__`` method that subclasses should
    not override; or if they do, the subclass should compute the same
    hash value.  This is so that sets with different implementations
    still hash to the same value, so they can be used interchangeably
    as dictionary keys.  (A similar constraint exists on the hash
    values for different types of numbers and strings.)

``MutableSet``

    This is a subclass of ``Set`` implementing additional operations
    to add and remove elements.  The supported methods have the
    semantics known from the ``set`` type in Python 2:

    ``.add(x)``
        Abstract method that adds the element ``x``, if it isn't
        already in the set.
    
    ``.remove(x)``

        Abstract method that removes the element ``x``; raises
        ``KeyError`` if ``x`` is not in the set.

    ``.discard(x)``
        Concrete method that removes the element ``x`` if it is
        a member of the set; implemented using ``__contains__``
        and ``remove``.

    ``.clear()``
        Abstract method that empties the set.  (Making this concrete
        would just add a slow, cumbersome default implementation.)

    XXX Should we support all the operations implemented by the Python
    2 ``set`` type?  I.e. union, update, __or__, __ror__, __ior__,
    intersection, intersection_update, __and__, __rand__, __iand__,
    difference, difference_update, __xor__, __rxor__, __ixor__,
    symmetric_difference, symmetric_difference_update, __sub__,
    __rsub__, __isub__.  Note that in Python 2, ``a.update(b)`` is not
    exactly the same as ``a |= b``, since ``update()`` takes any
    iterable for an argument, while ``|=`` requires another set;
    similar for the other operators.
    

Mappings
''''''''

These abstract classes represent various stages of mapping-ness.

XXX Do we need BasicMapping and IterableMapping?  Perhaps we should
just start with Mapping.

``BasicMapping``
    A subclass of ``Container`` defining the following methods:

    ``.__getitem__(key)``
        Abstract method that returns the value corresponding to
        ``key``, or raises ``KeyError``.  The implementation always
        raises ``KeyError``.

    ``.get(key, default=None)``
        Concrete method returning ``self[key]`` if this does not raise
        ``KeyError``, and the ``default`` value if it does.

    ``.__contains__()``
        Concrete method returning ``True`` if ``self[key]`` does not
        raise ``KeyError``, and ``False`` if it does.


``IterableMapping``
    A subclass of ``BasicMapping`` and ``Iterable``.  It defines no
    new methods.  Iterating over such an object should return all the
    valid keys (i.e. those keys for which ``.__getitem__()`` returns a
    value), once each, and nothing else.  It is possible that the
    iteration never ends.

``Mapping``
    A subclass of ``IterableMapping`` and ``Lengthy``.  It defines
    concrete methods ``__eq__``, ``keys``, ``items``, ``values``.  The
    lengh of such an object should equal to the number of elements
    returned by iterating over the object until the end of the
    iterator is reached.  Two mappings, even with different
    implementations, can be compared for equality, and are considered
    equal if and only iff their items compare equal when converted to
    sets.  The ``keys``, ``items`` and ``values`` methods return
    views; ``keys`` and ``items`` return ``Set`` views, ``values``
    returns a ``Container`` view.  The following invariant should
    hold: m.items() == set(zip(m.keys(), m.values())).

``HashableMapping``
    A subclass of ``Mapping`` and ``Hashable``.  The values should be
    instances of ``Hashable``.  The concrete ``__hash__`` method
    should be equal to ``hash(m.items())``.

``MutableMapping``
    A subclass of ``Mapping`` that also implements some standard
    mutating methods.  At least ``__setitem__``, ``__delitem__``,
    ``clear``, ``update``.  XXX Also pop, popitem, setdefault?

XXX Should probably say something about mapping view types, too.

Sequences
'''''''''

These abstract classes represent various stages of sequence-ness.

``Sequence``
    A subclass of ``Iterable``, ``Lengthy``, ``Container``.  It
    defines a new abstract method ``__getitem__`` that has a
    complicated signature: when called with an integer, it returns an
    element of the sequence or raises ``IndexError``; when called with
    a ``slice`` object, it returns another ``Sequence``.  The concrete
    ``__iter__`` method iterates over the elements using
    ``__getitem__`` with integer arguments 0, 1, and so on, until
    ``IndexError`` is raised.  The length should be equal to the
    number of values returned by the iterator.

    XXX Other candidate methods, which can all have default concrete
    implementations that only depend on ``__len__`` and
    ``__getitem__`` with an integer argument: __reversed__, index,
    count, __add__, __mul__, __eq__, __lt__, __le__.

``HashableSequence``
    A subclass of ``Sequence`` and ``Hashable``.  The concrete
    ``__hash__`` method should implements the hashing algorithms used
    by tuples in Python 2.

``MutableSequence``
    A subclass of ``Sequence`` adding some standard mutating methods.
    Abstract mutating methods: ``__setitem__`` (for integer indices as
    well as slices), ``__delitem__`` (ditto), ``insert``, ``add``,
    ``reverse``.  Concrete mutating methods: ``extend``, ``pop``,
    ``remove``.  Note: this does not define ``sort()`` -- that is only
    required to exist on genuine ``list`` instances.

    XXX What about ``+=`` and ``*=``?


ABCs for Numbers
----------------

XXX define: Number, Complex, Real, Rational, Integer.  Do we have a
use case for Cardinal (Integer >= 0)?  Do we need Indexable (converts
to Integer using __index__).


Guidelines for Writing ABCs
---------------------------

XXX Use @abstractmethod and Abstract base class; define abstract
methods that could be useful as an end point when called via a super
chain; define concrete methods that are very simple permutations of
abstract methods (e.g. Mapping.get); keep abstract classes small, one
per use case instead of one per concept.


References
==========

.. [1] An Introduction to ABC's, by Talin
   (http://mail.python.org/pipermail/python-3000/2007-April/006614.html)

.. [2] Incomplete implementation prototype, by GvR
   (http://svn.python.org/view/sandbox/trunk/abc/)


Copyright
=========

This document has been placed in the public domain.



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