python-peps/pep-0422.txt

PEP: 422
Title: Simple class initialisation hook
Version: $Revision$
Last-Modified: $Date$
Author: Nick Coghlan <ncoghlan@gmail.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 5-Jun-2012
Python-Version: 3.4
Post-History: 5-Jun-2012


Abstract
========

In Python 2, the body of a class definition could modify the way a class
was created (or simply arrange to run other code after the class was created)
by setting the ``__metaclass__`` attribute in the class body. While doing
this implicitly from called code required the use of an implementation detail
(specifically, ``sys._getframes()``), it could also be done explicitly in a
fully supported fashion (for example, by passing ``locals()`` to an
function that calculated a suitable ``__metaclass__`` value)

There is currently no corresponding mechanism in Python 3 that allows the
code executed in the class body to directly influence how the class object
is created. Instead, the class creation process is fully defined by the
class header, before the class body even begins executing.

This PEP proposes a mechanism that will once again allow the body of a
class definition to more directly influence the way a class is created
(albeit in a more constrained fashion), as well as replacing some current
uses of metaclasses with a simpler, easier to understand alternative.


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.


Proposal
========

This PEP proposes that a mechanism be added to Python 3 that meets the
following criteria:

1. 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
2. Integrates nicely with class inheritance structures (including mixins and
   multiple inheritance)
3. Integrates nicely with the implicit ``__class__`` reference and
   zero-argument ``super()`` syntax introduced by PEP 3135
4. Can be added to an existing base class without a significant risk of
   introducing backwards compatibility problems

One mechanism that would achieve this goal is to add a new class
initialisation hook, modelled directly on the existing instance
initialisation hook. However, the signature would be constrained to ensure
that correctly supporting multiple inheritance is kept as simple as possible.

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

   class Example:
       @classmethod
       def __init_class__(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 simple, decorator style invocation
           # ensures that this 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. Calling the hook automatically from
``type.__init__`` unfortunately doesn't work, as it would mean the
``__init_class__`` method would be unable to call any methods that relied
on the ``__class__`` reference (or used the zero-argument form of
``super()``).

If a metaclass wishes to block class initialisation for some reason, it
must arrange for ``cls.__init_class__`` to trigger ``AttributeError``.

This general proposal is not a new idea (it was first suggested `more than
10 years ago`_), but I believe the situation has changed sufficiently in
that time that the idea is worth reconsidering.


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


Replaces dynamic setting of ``__metaclass__``
---------------------------------------------

For use cases that didn't involve completely replacing the defined class,
Python 2 code that dynamically set ``__metaclass__`` can now dynamically
set ``__init_class__`` instead. For more advanced use cases, introduction of
an explicit metaclass will still be necessary in order to support Python 3.


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

Understanding Python's metaclass system requires a deep understanding of
the type system and the class construction process. This is legitimately
seen as confusing, 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.

Understanding the proposed class initialisation hook requires understanding
decorators and ordinary method inheritance, which is a much simpler prospect.


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

One of the big issues that makes library authors reluctant to use metaclasses
(even when it 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_class__`` 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, 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()``.


Alternatives
============


The Python 3 Status Quo
-----------------------

The Python 3 status quo already offers a great deal of flexibility. For
changes which only affect a single class definition and which can be
specified at the time the code is written, then class decorators can be
used to modify a class explicitly. Class decorators largely ignore class
inheritance and can make full use of methods that rely on the ``__class__``
reference being populated.

Using a custom metaclass provides the same level of power as it did in
Python 2. However, it's notable that, unlike class decorators, a metaclass
cannot call any methods that rely on the ``__class__`` reference, as that
reference is not populated until after the metaclass constructor returns
control to the class creation code.

One major use case for metaclasses actually closely resembles the use of
class decorators. It occurs whenever a metaclass has an implementation that
uses the following pattern::

    class Metaclass(type):
        def __new__(meta, *args, **kwds):
            cls = super(Metaclass, meta).__new__(meta, *args, **kwds)
            # Do something with cls
            return cls

The key difference between this pattern and a class decorator is that it
is automatically inherited by subclasses. However, it also comes with a
major disadvantage: Python does not allow you to inherit from classes with
unrelated metaclasses.

Thus, the status quo requires that developers choose between the following
two alternatives:

* Use a class decorator, meaning that behaviour is not inherited and must be
  requested explicitly on every subclass
* Use a metaclass, meaning that behaviour is inherited, but metaclass
  conflicts may make integration with other libraries and frameworks more
  difficult than it otherwise would be

If this PEP is ultimately rejected, then this is the existing design that
will remain in place by default.


Restoring the Python 2 metaclass hook
-------------------------------------

One simple alternative would be to restore support for a Python 2 style
``metaclass`` hook in the class body. This would be checked after the class
body was executed, potentially overwriting the metaclass hint provided in the
class header.

The main attraction of such an approach is that it would simplify porting
Python 2 applications that make use of this hook (especially those that do
so dynamically).

However, this approach does nothing to simplify the process of adding
*inherited* class definition time behaviour, nor does it interoperate
cleanly with the PEP 3135 ``__class__`` and ``super()`` semantics (as with
any metaclass based solution, the ``__metaclass__`` hook would have to run
before the ``__class__`` reference has been populated.


Dynamic class decorators
------------------------

The original version of this PEP was called "Dynamic class decorators" and
focused solely on a significantly more complicated proposal than that
presented in the current version.

As with the current version, it proposed that a new step be added to the
class creation process, after the metaclass invocation to construct the
class instance and before the application of lexical decorators. However,
instead of a simple process of calling a single class method that relies
on normal inheritance mechanisms, it proposed a far more complicated
procedure that walked the class MRO looking for decorators stored in
iterable ``__decorators__`` attributes.

Using the current version of the PEP, the scheme originally proposed could
be implemented as::

   class DynamicDecorators:
       @classmethod
       def __init_class__(cls):
           super(DynamicDecorators, cls).__init_class__()
           for entry in reversed(cls.mro()):
               decorators = entry.__dict__.get("__decorators__", ())
               for deco in reversed(decorators):
                   cls = deco(cls)

Any subclasses of this type would automatically have the contents of any
``__decorators__`` attributes processed and invoked.

The mechanism in the current PEP is considered superior, as many issues
to do with ordering and the same decorator being invoked multiple times
simple go away, as that kind of thing is taken care of through the use of an
ordinary class method invocation.


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


Copyright
=========

This document has been placed in the public domain.


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