python-peps/pep-0422.txt

PEP: 422
Title: Simple class initialisation hook
Version: $Revision$
Last-Modified: $Date$
Author: Nick Coghlan <ncoghlan@gmail.com>,
        Daniel Urban <urban.dani+py@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 a
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.

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 machiner).


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 can achieve this goal is to add a new class
initialisation hook, modelled directly on the existing instance
initialisation hook, but with the signature constrained to match that
of an ordinary class decorator.

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 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.

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

Note, that when ``__init_class__`` is called, the name of the class is not
bound to the new class object yet. 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.


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


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_class__`` 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.


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 ``__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 of ``__init_class__``, 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(Base):
       @classmethod
       def __init_class__(cls):
           # Process any classes later in the MRO
           try:
               mro_chain = super().__init_class__
           except AttributeError:
               pass
           else:
               mro_chain()
           # Process any __decorators__ attributes in the MRO
           for entry in reversed(cls.mro()):
               decorators = entry.__dict__.get("__decorators__", ())
               for deco in reversed(decorators):
                   cls = deco(cls)

Any subclasses of ``DynamicDecorators`` would then 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
just go away, as that kind of thing is taken care of through the use of an
ordinary class method invocation.


Automatic metaclass derivation
------------------------------

When no appropriate metaclass is found, it's theoretically possible to
automatically derive a metaclass for a new type based on the metaclass hint
and the metaclasses of the bases.

While adding such a mechanism would reduce the risk of spurious metaclass
conflicts, it would do nothing to improve integration with PEP 3135, would
not help with porting Python 2 code that set ``__metaclass__`` dynamically
and would not provide a more straightforward inherited mechanism for invoking
additional operations after the class invocation is complete.

In addition, there would still be a risk of metaclass conflicts in cases
where the base metaclasses were not written with multiple inheritance in
mind. In such situations, there's a chance of introducing latent defects
if one or more metaclasses are not invoked correctly.


Calling the new hook 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
``__init_class__`` 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.


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

The reference implementation has been posted to the `issue tracker`_.


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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