python-peps/pep-0487.txt

PEP: 487
Title: Simpler customisation of class creation
Version: $Revision$
Last-Modified: $Date$
Author: Martin Teichmann <lkb.teichmann@gmail.com>,
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 27-Feb-2015
Python-Version: 3.5
Post-History: 27-Feb-2015
Replaces: 422


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 ``__init_subclass__`` 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.


Connection to other PEP
=======================

This is a competing proposal to PEP 422 by Nick Coughlan and Daniel Urban.
It shares both most of the PEP text and proposed code, but has major
differences in how to achieve its goals.

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 3.5 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, and
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.

Those goals can be achieved by adding two functionalities:

1. A ``__init_subclass__`` hook that initializes all subclasses of a
   given class, and
2. A new keyword parameter ``namespace`` to the class creation statement,
   that gives an initialization of the namespace.

As an example, the first proposal looks as follows::

   class SpamBase:
       # this is implicitly a @classmethod
       def __init_subclass__(cls, ns, **kwargs):
           # This is invoked after a subclass is created, but before
           # explicit decorators are called.
           # The usual super() mechanisms are used to correctly support
           # multiple inheritance.
  # ns is the classes namespace
           # **kwargs are the keyword arguments to the subclasses'
           # class creation statement
           super().__init_subclass__(cls, ns, **kwargs)

   class Spam(SpamBase):
       pass
   # the new hook is called on Spam

To simplify the cooperative multiple inheritance case, ``object`` will gain
a default implementation of the hook that does nothing::

   class object:
       def __init_subclass__(cls, ns):
           pass

Note that this method has no keyword arguments, meaning that all
methods which are more specialized have to process all keyword
arguments.

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 for inclusion.

The second part of the proposal is to have a ``namespace`` keyword
argument to the class declaration statement. If present, its value
will be called without arguments to initialize a subclasses
namespace, very much like a metaclass ``__prepare__`` method would
do.

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 OrderedBase(namespace=collections.OrderedDict):
        pass

   class Ordered(OrderedBase):
        # 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.

Understanding the proposed implicit class initialization hook only requires
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 ``__init_subclass__`` 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.


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

Given that the method is called on already existing classes, 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 ``__init_subclass__`` 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.


A path of introduction into Python
==================================

Most of the benefits of this PEP can already be implemented using
a simple metaclass. For the ``__init_subclass__`` hook this works
all the way down to python 2.7, while the namespace needs python 3.0
to work. Such a class has been `uploaded to PyPI`_.

The only drawback of such a metaclass are the mentioned problems with
metaclasses and multiple inheritance. Two classes using such a
metaclass can only be combined, if they use exactly the same such
metaclass. This fact calls for the inclusion of such a class into the
standard library, let's call it ``SubclassMeta``, with a base class
using it called ``SublassInit``. Once all users use this standard
library metaclass, classes from different packages can easily be
combined.

But still such classes cannot be easily combined with other classes
using other metaclasses. Authors of metaclasses should bear that in
mind and inherit from the standard metaclass if it seems useful
for users of the metaclass to add more functionality. Ultimately,
if the need for combining with other metaclasses is strong enough,
the proposed functionality may be introduced into python's ``type``.

Those arguments strongly hint to the following procedure to include
the proposed functionality into python:

1. The metaclass implementing this proposal is put onto PyPI, so that
   it can be used and scrutinized.
2. Once the code is properly mature, it can be added to the python
   standard library. There should be a new module called
   ``metaclass`` which collects tools for metaclass authors, as well
   as a documentation of the best practices of how to write
   metaclasses.
3. If the need of combining this metaclass with other metaclasses is
   strong enough, it may be included into python itself.


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

This proposal has many usecases like the following. In the examples,
we still inherit from the ``SubclassInit`` base class. This would
become unnecessary once this PEP is included in Python directly.

Subclass registration
---------------------

Especially when writing a plugin system, one likes to register new
subclasses of a plugin baseclass. This can be done as follows::

   class PluginBase(SubclassInit):
       subclasses = []

       def __init_subclass__(cls, ns, **kwargs):
           super().__init_subclass__(ns, **kwargs)
           cls.subclasses.append(cls)

One should note that this also works nicely as a mixin class.

Trait descriptors
-----------------

There are many designs of python descriptors in the wild which, for
example, check boundaries of values. Often those "traits" need some support
of a metaclass to work. This is how this would look like with this
PEP::

   class Trait:
       def __get__(self, instance, owner):
           return instance.__dict__[self.key]

       def __set__(self, instance, value):
           instance.__dict__[self.key] = value

   class Int(Trait):
       def __set__(self, instance, value):
           # some boundary check code here
           super().__set__(instance, value)

   class HasTraits(SubclassInit):
       def __init_subclass__(cls, ns, **kwargs):
  super().__init_subclass__(ns, **kwargs)
           for k, v in ns.items():
               if isinstance(v, Trait):
                   v.key = k

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.


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

::

    class OrderedClassBase(namespace=collections.OrderedDict):
        pass

    class OrderedClass(OrderedClassBase):
        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


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


Calling the hook on the class itself
------------------------------------

Adding an ``__autodecorate__`` hook that would be called on the class
itself was the proposed idea of PEP 422.  Most examples work the same
way or even better if the hook is called on the subclass. In general,
it is much easier to explicitly call the hook on the class in which it
is defined (to opt-in to such a behavior) than to opt-out, meaning
that one does not want the hook to be called on the class it is
defined in.

This becomes most evident if the class in question is designed as a
mixin: it is very unlikely that the code of the mixin is to be
executed for the mixin class itself, as it is not supposed to be a
complete class on its own.

The original proposal also made major changes in the class
initialization process, rendering it impossible to back-port the
proposal to older python versions.


Other variants of calling the hook
----------------------------------

Other names for the hook were presented, namely ``__decorate__`` or
``__autodecorate__``. This proposal opts for ``__init_subclass__`` as
it is very close to the ``__init__`` method, just for the subclass,
while it is not very close to decorators, as it does not return the
class.


Requiring an explicit decorator on ``__init_subclass__``
--------------------------------------------------------

One could require the explicit use of ``@classmethod`` on the
``__init_subclass__`` 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, PEP 422 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.


Possible Extensions
===================

Some extensions to this PEP are imaginable, which are postponed to a
later pep:

* A ``__new_subclass__`` method could be defined which acts like a
  ``__new__`` for classes. This would be very close to
  ``__autodecorate__`` in PEP 422.
* ``__subclasshook__`` could be made a classmethod in a class instead
  of a method in the metaclass.

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

.. _uploaded to PyPI:
   https://pypi.python.org/pypi/metaclass

Copyright
=========

This document has been placed in the public domain.



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