python-peps/pep-0422.txt

PEP: 422
Title: Simpler customisation of class creation
Version: $Revision$
Last-Modified: $Date$
Author: Nick Coghlan <ncoghlan@gmail.com>,
        Daniel Urban <urban.dani+py@gmail.com>
Status: Deferred
Type: Standards Track
Content-Type: text/x-rst
Created: 5-Jun-2012
Python-Version: 3.5
Post-History: 5-Jun-2012, 10-Feb-2013


Abstract
========

Currently, customising class creation requires the use of a custom metaclass.
This custom metaclass then persists for the entire lifecycle of the class,
creating the potential for spurious metaclass conflicts.

This PEP proposes to instead support a wide range of customisation
scenarios through a new ``namespace`` parameter in the class header, and
a new ``__autodecorate__`` hook in the class body.

The new mechanism should be easier to understand and use than
implementing a custom metaclass, and thus should provide a gentler
introduction to the full power Python's metaclass machinery.

PEP Deferral
============

Deferred until 3.5 at the earliest. The last review raised a few interesting
points that I (Nick) need to consider further before proposing it for
inclusion, and that's not going to happen in the 3.4 timeframe.


Background
==========

For an already created class ``cls``, the term "metaclass" has a clear
meaning: it is the value of ``type(cls)``.

*During* class creation, it has another meaning: it is also used to refer to
the metaclass hint that may be provided as part of the class definition.
While in many cases these two meanings end up referring to one and the same
object, there are two situations where that is not the case:

* If the metaclass hint refers to an instance of ``type``, then it is
  considered as a candidate metaclass along with the metaclasses of all of
  the parents of the class being defined. If a more appropriate metaclass is
  found amongst the candidates, then it will be used instead of the one
  given in the metaclass hint.
* Otherwise, an explicit metaclass hint is assumed to be a factory function
  and is called directly to create the class object. In this case, the final
  metaclass will be determined by the factory function definition. In the
  typical case (where the factory functions just calls ``type``, or, in
  Python 3.3 or later, ``types.new_class``) the actual metaclass is then
  determined based on the parent classes.

It is notable that only the actual metaclass is inherited - a factory
function used as a metaclass hook sees only the class currently being
defined, and is not invoked for any subclasses.

In Python 3, the metaclass hint is provided using the ``metaclass=Meta``
keyword syntax in the class header. This allows the ``__prepare__`` method
on the metaclass to be used to create the ``locals()`` namespace used during
execution of the class body (for example, specifying the use of
``collections.OrderedDict`` instead of a regular ``dict``).

In Python 2, there was no ``__prepare__`` method (that API was added for
Python 3 by PEP 3115). Instead, a class body could set the ``__metaclass__``
attribute, and the class creation process would extract that value from the
class namespace to use as the metaclass hint. There is `published code`_ that
makes use of this feature.

Another new feature in Python 3 is the zero-argument form of the ``super()``
builtin, introduced by PEP 3135. This feature uses an implicit ``__class__``
reference to the class being defined to replace the "by name" references
required in Python 2. Just as code invoked during execution of a Python 2
metaclass could not call methods that referenced the class by name (as the
name had not yet been bound in the containing scope), similarly, Python 3
metaclasses cannot call methods that rely on the implicit ``__class__``
reference (as it is not populated until after the metaclass has returned
control to the class creation machinery).

Finally, when a class uses a custom metaclass, it can pose additional
challenges to the use of multiple inheritance, as a new class cannot
inherit from parent classes with unrelated metaclasses. This means that
it is impossible to add a metaclass to an already published class: such
an addition is a backwards incompatible change due to the risk of metaclass
conflicts.


Proposal
========

This PEP proposes that a new mechanism to customise class creation be
added to Python 3.4 that meets the following criteria:

1. Integrates nicely with class inheritance structures (including mixins and
   multiple inheritance)
2. Integrates nicely with the implicit ``__class__`` reference and
   zero-argument ``super()`` syntax introduced by PEP 3135
3. Can be added to an existing base class without a significant risk of
   introducing backwards compatibility problems
4. Restores the ability for class namespaces to have some influence on the
   class creation process (above and beyond populating the namespace itself),
   but potentially without the full flexibility of the Python 2 style
   ``__metaclass__`` hook

One mechanism that can achieve this goal is to add a new implicit class
decoration hook, modelled directly on the existing explicit class
decorators, but defined in the class body or in a parent class, rather than
being part of the class definition header.

Specifically, it is proposed that class definitions be able to provide a
class initialisation hook as follows::

   class Example:
       def __autodecorate__(cls):
           # This is invoked after the class is created, but before any
           # explicit decorators are called
           # The usual super() mechanisms are used to correctly support
           # multiple inheritance. The class decorator style signature helps
           # ensure that invoking the parent class is as simple as possible.

If present on the created object, this new hook will be called by the class
creation machinery *after* the ``__class__`` reference has been initialised.
For ``types.new_class()``, it will be called as the last step before
returning the created class object. ``__autodecorate__`` is implicitly
converted to a class method when the class is created (prior to the hook
being invoked).

If a metaclass wishes to block implicit class decoration for some reason, it
must arrange for ``cls.__autodecorate__`` to trigger ``AttributeError``.

Note, that when ``__autodecorate__`` is called, the name of the class is not
yet bound to the new class object. As a consequence, the two argument form
of ``super()`` cannot be used to call methods (e.g., ``super(Example, cls)``
wouldn't work in the example above). However, the zero argument form of
``super()`` works as expected, since the ``__class__`` reference is already
initialised.

This general proposal is not a new idea (it was first suggested for
inclusion in the language definition `more than 10 years ago`_, and a
similar mechanism has long been supported by `Zope's ExtensionClass`_),
but the situation has changed sufficiently in recent years that
the idea is worth reconsidering.

In addition, the introduction of the metaclass ``__prepare__`` method in PEP
3115 allows a further enhancement that was not possible in Python 2: this
PEP also proposes that ``type.__prepare__`` be updated to accept a factory
function as a ``namespace`` keyword-only argument. If present, the value
provided as the ``namespace`` argument will be called without arguments
to create the result of ``type.__prepare__`` instead of using a freshly
created dictionary instance. For example, the following will use
an ordered dictionary as the class namespace::

   class OrderedExample(namespace=collections.OrderedDict):
       def __autodecorate__(cls):
           # cls.__dict__ is still a read-only proxy to the class namespace,
           # but the underlying storage is an OrderedDict instance

.. note::

    This PEP, along with the existing ability to use  __prepare__ to share a
    single namespace amongst multiple class objects, highlights a possible
    issue with the attribute lookup caching: when the underlying mapping is
    updated by other means, the attribute lookup cache is not invalidated
    correctly (this is a key part of the reason class ``__dict__`` attributes
    produce a read-only view of the underlying storage).

    Since the optimisation provided by that cache is highly desirable,
    the use of a preexisting namespace as the class namespace may need to
    be declared as officially unsupported (since the observed behaviour is
    rather strange when the caches get out of sync).


Key Benefits
============


Easier use of custom namespaces for a class
-------------------------------------------

Currently, to use a different type (such as ``collections.OrderedDict``) for
a class namespace, or to use a pre-populated namespace, it is necessary to
write and use a custom metaclass. With this PEP, using a custom namespace
becomes as simple as specifying an appropriate factory function in the
class header.


Easier inheritance of definition time behaviour
-----------------------------------------------

Understanding Python's metaclasses requires a deep understanding of
the type system and the class construction process. This is legitimately
seen as challenging, due to the need to keep multiple moving parts (the code,
the metaclass hint, the actual metaclass, the class object, instances of the
class object) clearly distinct in your mind. Even when you know the rules,
it's still easy to make a mistake if you're not being extremely careful.
An earlier version of this PEP actually included such a mistake: it
stated "subclass of type" for a constraint that is actually "instance of
type".

Understanding the proposed class initialisation hook only requires
understanding decorators and ordinary method inheritance, which isn't
quite as daunting a task. The new hook provides a more gradual path
towards understanding all of the phases involved in the class definition
process.


Reduced chance of metaclass conflicts
-------------------------------------

One of the big issues that makes library authors reluctant to use metaclasses
(even when they would be appropriate) is the risk of metaclass conflicts.
These occur whenever two unrelated metaclasses are used by the desired
parents of a class definition. This risk also makes it very difficult to
*add* a metaclass to a class that has previously been published without one.

By contrast, adding an ``__autodecorate__`` method to an existing type poses
a similar level of risk to adding an ``__init__`` method: technically, there
is a risk of breaking poorly implemented subclasses, but when that occurs,
it is recognised as a bug in the subclass rather than the library author
breaching backwards compatibility guarantees. In fact, due to the constrained
signature of ``__autodecorate__``, the risk in this case is actually even
lower than in the case of ``__init__``.


Integrates cleanly with \PEP 3135
---------------------------------

Unlike code that runs as part of the metaclass, code that runs as part of
the new hook will be able to freely invoke class methods that rely on the
implicit ``__class__`` reference introduced by PEP 3135, including methods
that use the zero argument form of ``super()``.


Replaces many use cases for dynamic setting of ``__metaclass__``
-----------------------------------------------------------------

For use cases that don't involve completely replacing the defined class,
Python 2 code that dynamically set ``__metaclass__`` can now dynamically
set ``__autodecorate__`` instead. For more advanced use cases, introduction of
an explicit metaclass (possibly made available as a required base class) will
still be necessary in order to support Python 3.


Design Notes
============


Determining if the class being decorated is the base class
----------------------------------------------------------

In the body of an ``__autodecorate__`` method, as in any other class method,
``__class__`` will be bound to the class declaring the method, while the
value passed in may be a subclass.

This makes it relatively straightforward to skip processing the base class
if necessary::

   class Example:
       def __autodecorate__(cls):
           # Don't process the base class
           if cls is __class__:
               return
           # Process subclasses here


New Ways of Using Classes
=========================

The new ``namespace`` keyword in the class header enables a number of
interesting options for controlling the way a class is initialised,
including some aspects of the object models of both Javascript and Ruby.

All of the examples below are actually possible today through the use of a
custom metaclass::

    class CustomNamespace(type):
        @classmethod
        def __prepare__(meta, name, bases, *, namespace=None, **kwds):
            parent_namespace = super().__prepare__(name, bases, **kwds)
            return namespace() if namespace is not None else parent_namespace
        def __new__(meta, name, bases, ns, *, namespace=None, **kwds):
            return super().__new__(meta, name, bases, ns, **kwds)
        def __init__(cls, name, bases, ns, *, namespace=None, **kwds):
            return super().__init__(name, bases, ns, **kwds)

The advantage of implementing the new keyword directly in
``type.__prepare__`` is that the *only* persistent effect is then
the change in the underlying storage of the class attributes. The metaclass
of the class remains unchanged, eliminating many of the drawbacks
typically associated with these kinds of customisations.


Order preserving classes
------------------------

::

    class OrderedClass(namespace=collections.OrderedDict):
        a = 1
        b = 2
        c = 3


Prepopulated namespaces
-----------------------

::

    seed_data = dict(a=1, b=2, c=3)
    class PrepopulatedClass(namespace=seed_data.copy):
        pass


Cloning a prototype class
-------------------------

::

    class NewClass(namespace=Prototype.__dict__.copy):
        pass


Extending a class
-----------------

.. note:: Just because the PEP makes it *possible* to do this relatively,
   cleanly doesn't mean anyone *should* do this!

::

    from collections import MutableMapping

    # The MutableMapping + dict combination should give something that
    # generally behaves correctly as a mapping, while still being accepted
    # as a class namespace
    class ClassNamespace(MutableMapping, dict):
        def __init__(self, cls):
            self._cls = cls
        def __len__(self):
            return len(dir(self._cls))
        def __iter__(self):
            for attr in dir(self._cls):
                yield attr
        def __contains__(self, attr):
            return hasattr(self._cls, attr)
        def __getitem__(self, attr):
            return getattr(self._cls, attr)
        def __setitem__(self, attr, value):
            setattr(self._cls, attr, value)
        def __delitem__(self, attr):
            delattr(self._cls, attr)

    def extend(cls):
        return lambda: ClassNamespace(cls)

    class Example:
        pass

    class ExtendedExample(namespace=extend(Example)):
        a = 1
        b = 2
        c = 3

    >>> Example.a, Example.b, Example.c
    (1, 2, 3)


Rejected Design Options
=======================


Calling ``__autodecorate__`` from ``type.__init__``
---------------------------------------------------

Calling the new hook automatically from ``type.__init__``, would achieve most
of the goals of this PEP. However, using that approach would mean that
``__autodecorate__`` implementations would be unable to call any methods that
relied on the ``__class__`` reference (or used the zero-argument form of
``super()``), and could not make use of those features themselves.


Calling the automatic decoration hook ``__init_class__``
--------------------------------------------------------

Earlier versions of the PEP used the name ``__init_class__`` for the name
of the new hook. There were three significant problems with this name:

* it was hard to remember if the correct spelling was ``__init_class__`` or
  ``__class_init__``
* the use of "init" in the name suggested the signature should match that
  of ``type.__init__``, which is not the case
* the use of "init" in the name suggested the method would be run as part
  of class construction, which is not the case

The new name ``__autodecorate__`` was chosen to make it clear that the new
initialisation hook is most usefully thought of as an implicitly invoked
class decorator, rather than as being like an ``__init__`` method.


Requiring an explicit decorator on ``__autodecorate__``
-------------------------------------------------------

Originally, this PEP required the explicit use of ``@classmethod`` on the
``__autodecorate__`` decorator. It was made implicit since there's no
sensible interpretation for leaving it out, and that case would need to be
detected anyway in order to give a useful error message.

This decision was reinforced after noticing that the user experience of
defining ``__prepare__`` and forgetting the ``@classmethod`` method
decorator is singularly incomprehensible (particularly since PEP 3115
documents it as an ordinary method, and the current documentation doesn't
explicitly say anything one way or the other).


Passing in the namespace directly rather than a factory function
----------------------------------------------------------------

At one point, this PEP proposed that the class namespace be passed
directly as a keyword argument, rather than passing a factory function.
However, this encourages an unsupported behaviour (that is, passing the
same namespace to multiple classes, or retaining direct write access
to a mapping used as a class namespace), so the API was switched to
the factory function version.


Reference Implementation
========================

A reference implementation for ``__autodecorate__`` has been posted to the
`issue tracker`_. It uses the original ``__init_class__`` naming and does
not yet include the new ``namespace`` parameter for ``type.__prepare__``.

TODO
====

* address the 5 points in http://mail.python.org/pipermail/python-dev/2013-February/123970.html

References
==========

.. _published code:
   http://mail.python.org/pipermail/python-dev/2012-June/119878.html

.. _more than 10 years ago:
   http://mail.python.org/pipermail/python-dev/2001-November/018651.html

.. _Zope's ExtensionClass:
   http://docs.zope.org/zope_secrets/extensionclass.html

.. _issue tracker:
   http://bugs.python.org/issue17044

Copyright
=========

This document has been placed in the public domain.


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