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PEP 487: New version from Martin

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Nick Coghlan 2016-02-06 15:26:52 +10:00
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@ -7,8 +7,8 @@ Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 27-Feb-2015
Python-Version: 3.5
Post-History: 27-Feb-2015
Python-Version: 3.6
Post-History: 27-Feb-2015, 5-Feb-2016
Replaces: 422
@ -20,127 +20,80 @@ 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.
scenarios through a new ``__init_subclass__`` hook in the class body,
a hook to initialize descriptors, and a way to keep the order in which
attributes are defined.
Those hooks should at first be defined in a metaclass in the standard
library, with the option that this metaclass eventually becomes the
default ``type`` metaclass.
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.
Metaclasses are a powerful tool to customize class creation. They have,
however, the problem that there is no automatic way to combine metaclasses.
If one wants to use two metaclasses for a class, a new metaclass combining
those two needs to be created, typically manually.
This need often occurs as a surprise to a user: inheriting from two base
classes coming from two different libraries suddenly raises the necessity
to manually create a combined metaclass, where typically one is not
interested in those details about the libraries at all. This becomes
even worse if one library starts to make use of a metaclass which it
has not done before. While the library itself continues to work perfectly,
suddenly every code combining those classes with classes from another library
fails.
Proposal
========
This PEP proposes that a new mechanism to customise class creation be
added to Python 3.5 that meets the following criteria:
While there are many possible ways to use a metaclass, the vast majority
of use cases falls into just three categories: some initialization code
running after class creation, the initalization of descriptors and
keeping the order in which class attributes were defined.
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 three use cases can easily be performed by just one metaclass. If
this metaclass is put into the standard library, and all libraries that
wish to customize class creation use this very metaclass, no combination
of metaclasses is necessary anymore.
Those goals can be achieved by adding two functionalities:
The three use cases are achieved as follows:
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.
1. The metaclass contains an ``__init_subclass__`` hook that initializes
all subclasses of a given class,
2. the metaclass calls an ``__init_descriptor__`` hook for all descriptors
defined in the class, and
3. an ``__attribute_order__`` tuple is left in the class in order to inspect
the order in which attributes were defined.
As an example, the first proposal looks as follows::
For ease of use, a base class ``SubclassInit`` is defined, which uses said
metaclass and contains an empty stub for the hook described for use case 1.
class SpamBase:
As an example, the first use case looks as follows::
class SpamBase(SubclassInit):
# this is implicitly a @classmethod
def __init_subclass__(cls, ns, **kwargs):
def __init_subclass__(cls, **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)
super().__init_subclass__(cls, **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
The base class ``SubclassInit`` contains an empty ``__init_subclass__``
method which serves as an endpoint for cooperative multiple inheritance.
Note that this method has no keyword arguments, meaning that all
methods which are more specialized have to process all keyword
arguments.
@ -151,58 +104,46 @@ 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.
The second part of the proposal adds an ``__init_descriptor__``
initializer for descriptors. Descriptors are defined in the body of a
class, but they do not know anything about that class, they do not
even know the name they are accessed with. They do get to know their
owner once ``__get__`` is called, but still they do not know their
name. This is unfortunate, for example they cannot put their
associated value into their object's ``__dict__`` under their name,
since they do not know that name. This problem has been solved many
times, and is one of the most important reasons to have a metaclass in
a library. While it would be easy to implement such a mechanism using
the first part of the proposal, it makes sense to have one solution
for this problem for everyone.
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::
To give an example of its usage, imagine a descriptor representing weak
referenced values (this is an insanely simplified, yet working example)::
class OrderedBase(namespace=collections.OrderedDict):
pass
import weakref
class Ordered(OrderedBase):
# cls.__dict__ is still a read-only proxy to the class namespace,
# but the underlying storage is an OrderedDict instance
class WeakAttribute:
def __get__(self, instance, owner):
return instance.__dict__[self.name]
def __set__(self, instance, value):
instance.__dict__[self.name] = weakref.ref(value)
.. note::
# this is the new initializer:
def __init_descriptor__(self, owner, name):
self.name = name
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).
The third part of the proposal is to leave a tuple called
``__attribute_order__`` in the class that contains the order in which
the attributes were defined. This is a very common usecase, many
libraries use an ``OrderedDict`` to store this order. This is a very
simple way to achieve the same goal.
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
-----------------------------------------------
@ -235,40 +176,20 @@ 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
all the way down to Python 2.7, while the attribute order 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
standard library, let's call it ``SubclassMeta``, with the base class
using it called ``SubclassInit``. Once all users use this standard
library metaclass, classes from different packages can easily be
combined.
@ -277,21 +198,32 @@ 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``.
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:
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
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.
strong enough, it may be included into Python itself.
While the metaclass is still in the standard library and not in the
language, it may still clash with other metaclasses. The most
prominent metaclass in use is probably ABCMeta. It is also a
particularly good example for the need of combining metaclasses. For
users who want to define a ABC with subclass initialization, we should
support a ``ABCSubclassInit`` class, or let ABCMeta inherit from this
PEP's metaclass.
Extensions written in C or C++ also often define their own metaclass.
It would be very useful if those could also inherit from the metaclass
defined here, but this is probably not possible.
New Ways of Using Classes
=========================
@ -309,8 +241,8 @@ 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)
def __init_subclass__(cls, **kwargs):
super().__init_subclass__(**kwargs)
cls.subclasses.append(cls)
One should note that this also works nicely as a mixin class.
@ -318,7 +250,7 @@ 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
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::
@ -330,55 +262,14 @@ PEP::
def __set__(self, instance, value):
instance.__dict__[self.key] = value
def __init_descriptor__(self, owner, name):
self.key = name
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
=======================
@ -402,7 +293,7 @@ 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.
proposal to older Python versions.
Other variants of calling the hook
@ -430,28 +321,47 @@ 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
----------------------------------------------------------------
Defining arbitrary namespaces
-----------------------------
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.
PEP 422 defined a generic way to add arbitrary namespaces for class
definitions. This approach is much more flexible than just leaving
the definition order in a tuple. The ``__prepare__`` method in a metaclass
supports exactly this behavior. But given that effectively
the only use cases that could be found out in the wild were the
``OrderedDict`` way of determining the attribute order, it seemed
reasonable to only support this special case.
The metaclass described in this PEP has been designed to be very simple
such that it could be reasonably made the default metaclass. This was
especially important when designing the attribute order functionality:
This was a highly demanded feature and has been enabled through the
``__prepare__`` method of metaclasses. This method can be abused in
very weird ways, making it hard to correctly maintain this feature in
CPython. This is why it has been proposed to deprecated this feature,
and instead use ``OrderedDict`` as the standard namespace, supporting
the most important feature while dropping most of the complexity. But
this would have meant that ``OrderedDict`` becomes a language builtin
like dict and set, and not just a standard library class. The choice
of the ``__attribute_order__`` tuple is a much simpler solution to the
problem.
A more ``__new__``-like hook
----------------------------
In PEP 422 the hook worked more like the ``__new__`` method than the
``__init__`` method, meaning that it returned a class instead of
modifying one. This allows a bit more flexibility, but at the cost
of much harder implementation and undesired side effects.
Possible Extensions
===================
History
=======
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.
This used to be 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. In the meantime, PEP 422
has been withdrawn favouring this approach.
References
==========