321 lines
11 KiB
Plaintext
321 lines
11 KiB
Plaintext
PEP: 520
|
||
Title: Preserving Class Attribute Definition Order
|
||
Version: $Revision$
|
||
Last-Modified: $Date$
|
||
Author: Eric Snow <ericsnowcurrently@gmail.com>
|
||
Status: Draft
|
||
Type: Standards Track
|
||
Content-Type: text/x-rst
|
||
Created: 7-Jun-2016
|
||
Python-Version: 3.6
|
||
Post-History: 7-Jun-2016, 11-Jun-2016, 20-Jun-2016
|
||
|
||
|
||
Abstract
|
||
========
|
||
|
||
When a class is defined using a ``class`` statement, the class body is
|
||
executed within a namespace. After the execution completes, that
|
||
namespace is copied into new ``dict`` and the original definition
|
||
namespace is discarded. The new copy is stored away as the class's
|
||
namespace and is exposed as ``__dict__`` through a read-only proxy.
|
||
|
||
This PEP preserves the order in which the attributes in the definition
|
||
namespace were added to it, before that namespace is discarded. This
|
||
means it reflects the definition order of the class body. That order
|
||
will now be preserved in the ``__definition_order__`` attribute of the
|
||
class. This allows introspection of the original definition order,
|
||
e.g. by class decorators.
|
||
|
||
Additionally, this PEP changes the default class definition namespace
|
||
to ``OrderedDict``. The long-lived class namespace (``__dict__``) will
|
||
remain a ``dict``.
|
||
|
||
|
||
Motivation
|
||
==========
|
||
|
||
Currently Python does not preserve the order in which attributes are
|
||
added to the class definition namespace. The namespace used during
|
||
execution of a class body defaults to ``dict``. If the metaclass
|
||
defines ``__prepare__()`` then the result of calling it is used. Thus,
|
||
before this PEP, to access your class definition namespace you must
|
||
use ``OrderedDict`` along with a metaclass. Then you must preserve the
|
||
definition order (from the ``OrderedDict``) yourself. This has a
|
||
couple of problems.
|
||
|
||
First, it requires the use of a metaclass. Metaclasses introduce an
|
||
extra level of complexity to code and in some cases (e.g. conflicts)
|
||
are a problem. So reducing the need for them is worth doing when the
|
||
opportunity presents itself. PEP 422 and PEP 487 discuss this at
|
||
length. Given that we now have a C implementation of ``OrderedDict``
|
||
and that ``OrderedDict`` is the common use case for ``__prepare__()``,
|
||
we have such an opportunity by defaulting to ``OrderedDict``.
|
||
|
||
Second, only classes that opt in to using the ``OrderedDict``-based
|
||
metaclass will have access to the definition order. This is problematic
|
||
for cases where universal access to the definition order is important.
|
||
One of the original motivating use cases for this PEP is generic class
|
||
decorators that make use of the definition order.
|
||
|
||
|
||
Specification
|
||
=============
|
||
|
||
Part 1:
|
||
|
||
* the order in which class attributes are defined is preserved in the
|
||
new ``__definition_order__`` attribute on each class
|
||
* "dunder" attributes (e.g. ``__init__``, ``__module__``) are ignored
|
||
* ``__definition_order__`` is a ``tuple`` (or ``None``)
|
||
* ``__definition_order__`` is a read-only attribute
|
||
* ``__definition_order__`` is always set:
|
||
|
||
1. if ``__definition_order__`` is defined in the class body then it
|
||
must be a ``tuple`` of identifiers or ``None``; any other value
|
||
will result in ``TypeError``
|
||
2. classes that do not have a class definition (e.g. builtins) have
|
||
their ``__definition_order__`` set to ``None``
|
||
3. classes for which `__prepare__()`` returned something other than
|
||
``OrderedDict`` (or a subclass) have their ``__definition_order__``
|
||
set to ``None`` (except where #1 applies)
|
||
|
||
Part 2:
|
||
|
||
* the default class *definition* namespace is now ``OrderdDict``
|
||
|
||
The following code demonstrates roughly equivalent semantics for the
|
||
default behavior::
|
||
|
||
class Meta(type):
|
||
@classmethod
|
||
def __prepare__(cls, *args, **kwargs):
|
||
return OrderedDict()
|
||
|
||
class Spam(metaclass=Meta):
|
||
ham = None
|
||
eggs = 5
|
||
__definition_order__ = tuple(k for k in locals()
|
||
if not (k.startswith('__') and
|
||
k.endswith('__')))
|
||
|
||
Note that [pep487_] proposes a similar solution, albeit as part of a
|
||
broader proposal.
|
||
|
||
Why a tuple?
|
||
------------
|
||
|
||
Use of a tuple reflects the fact that we are exposing the order in
|
||
which attributes on the class were *defined*. Since the definition
|
||
is already complete by the time ``__definition_order__`` is set, the
|
||
content and order of the value won't be changing. Thus we use a type
|
||
that communicates that state of immutability.
|
||
|
||
Why a read-only attribute?
|
||
--------------------------
|
||
|
||
As with the use of tuple, making ``__definition_order__`` a read-only
|
||
attribute communicates the fact that the information it represents is
|
||
complete. Since it represents the state of a particular one-time event
|
||
(execution of the class definition body), allowing the value to be
|
||
replaced would reduce confidence that the attribute corresponds to the
|
||
original class body.
|
||
|
||
If a use case for a writable (or mutable) ``__definition_order__``
|
||
arises, the restriction may be loosened later. Presently this seems
|
||
unlikely and furthermore it is usually best to go immutable-by-default.
|
||
|
||
Note that ``__definition_order__`` is centered on the class definition
|
||
body. The use cases for dealing with the class namespace (``__dict__``)
|
||
post-definition are a separate matter. ``__definition_order__`` would
|
||
be a significantly misleading name for a feature focused on more than
|
||
class definition.
|
||
|
||
See [nick_concern_] for more discussion.
|
||
|
||
Why ignore "dunder" names?
|
||
--------------------------
|
||
|
||
Names starting and ending with "__" are reserved for use by the
|
||
interpreter. In practice they should not be relevant to the users of
|
||
``__definition_order__``. Instead, for nearly everyone they would only
|
||
be clutter, causing the same extra work for everyone.
|
||
|
||
Why None instead of an empty tuple?
|
||
-----------------------------------
|
||
|
||
A key objective of adding ``__definition_order__`` is to preserve
|
||
information in class definitions which was lost prior to this PEP.
|
||
One consequence is that ``__definition_order__`` implies an original
|
||
class definition. Using ``None`` allows us to clearly distinquish
|
||
classes that do not have a definition order. An empty tuple clearly
|
||
indicates a class that came from a definition statement but did not
|
||
define any attributes there.
|
||
|
||
Why None instead of not setting the attribute?
|
||
----------------------------------------------
|
||
|
||
The absence of an attribute requires more complex handling than ``None``
|
||
does for consumers of ``__definition_order__``.
|
||
|
||
Why constrain manually set values?
|
||
----------------------------------
|
||
|
||
If ``__definition_order__`` is manually set in the class body then it
|
||
will be used. We require it to be a tuple of identifiers (or ``None``)
|
||
so that consumers of ``__definition_order__`` may have a consistent
|
||
expectation for the value. That helps maximize the feature's
|
||
usefulness.
|
||
|
||
We could also also allow an arbitrary iterable for a manually set
|
||
``__definition_order__`` and convert it into a tuple. However, not
|
||
all iterables infer a definition order (e.g. ``set``). So we opt in
|
||
favor of requiring a tuple.
|
||
|
||
Why is __definition_order__ even necessary?
|
||
-------------------------------------------
|
||
|
||
Since the definition order is not preserved in ``__dict__``, it is
|
||
lost once class definition execution completes. Classes *could*
|
||
explicitly set the attribute as the last thing in the body. However,
|
||
then independent decorators could only make use of classes that had done
|
||
so. Instead, ``__definition_order__`` preserves this one bit of info
|
||
from the class body so that it is universally available.
|
||
|
||
|
||
Support for C-API Types
|
||
=======================
|
||
|
||
Arguably, most C-defined Python types (e.g. built-in, extension modules)
|
||
have a roughly equivalent concept of a definition order. So conceivably
|
||
``__definition_order__`` could be set for such types automatically. This
|
||
PEP does not introduce any such support. However, it does not prohibit
|
||
it either.
|
||
|
||
|
||
Compatibility
|
||
=============
|
||
|
||
This PEP does not break backward compatibility, except in the case that
|
||
someone relies *strictly* on ``dict`` as the class definition namespace.
|
||
This shouldn't be a problem since ``issubclass(OrderedDict, dict)`` is
|
||
true.
|
||
|
||
|
||
Changes
|
||
=============
|
||
|
||
In addition to the class syntax, the following expose the new behavior:
|
||
|
||
* builtins.__build_class__
|
||
* types.prepare_class
|
||
* types.new_class
|
||
|
||
|
||
Other Python Implementations
|
||
============================
|
||
|
||
Pending feedback, the impact on Python implementations is expected to
|
||
be minimal. If a Python implementation cannot support switching to
|
||
`OrderedDict``-by-default then it can always set ``__definition_order__``
|
||
to ``None``.
|
||
|
||
|
||
Implementation
|
||
==============
|
||
|
||
The implementation is found in the tracker. [impl_]
|
||
|
||
|
||
Alternatives
|
||
============
|
||
|
||
cls.__dict__ as OrderedDict
|
||
-------------------------------
|
||
|
||
Instead of storing the definition order in ``__definition_order__``,
|
||
the now-ordered definition namespace could be copied into a new
|
||
``OrderedDict``. This would then be used as the mapping proxied as
|
||
``__dict__``. Doing so would mostly provide the same semantics.
|
||
|
||
However, using ``OrderedDict`` for ``__dict__`` would obscure the
|
||
relationship with the definition namespace, making it less useful.
|
||
Additionally, doing this would require significant changes to the
|
||
semantics of the concrete ``dict`` C-API.
|
||
|
||
A "namespace" Keyword Arg for Class Definition
|
||
----------------------------------------------
|
||
|
||
PEP 422 introduced a new "namespace" keyword arg to class definitions
|
||
that effectively replaces the need to ``__prepare__()``. [pep422_]
|
||
However, the proposal was withdrawn in favor of the simpler PEP 487.
|
||
|
||
A stdlib Metaclass that Implements __prepare__() with OrderedDict
|
||
-----------------------------------------------------------------
|
||
|
||
This has all the same problems as writing your own metaclass. The
|
||
only advantage is that you don't have to actually write this
|
||
metaclass. So it doesn't offer any benefit in the context of this
|
||
PEP.
|
||
|
||
Set __definition_order__ at Compile-time
|
||
----------------------------------------
|
||
|
||
Each class's ``__qualname__`` is determined at compile-time.
|
||
This same concept could be applied to ``__definition_order__``.
|
||
The result of composing ``__definition_order__`` at compile-time
|
||
would be nearly the same as doing so at run-time.
|
||
|
||
Comparative implementation difficulty aside, the key difference
|
||
would be that at compile-time it would not be practical to
|
||
preserve definition order for attributes that are set dynamically
|
||
in the class body (e.g. ``locals()[name] = value``). However,
|
||
they should still be reflected in the definition order. One
|
||
posible resolution would be to require class authors to manually
|
||
set ``__definition_order__`` if they define any class attributes
|
||
dynamically.
|
||
|
||
Ultimately, the use of ``OrderedDict`` at run-time or compile-time
|
||
discovery is almost entirely an implementation detail.
|
||
|
||
|
||
References
|
||
==========
|
||
|
||
.. [impl] issue #24254
|
||
(https://bugs.python.org/issue24254)
|
||
|
||
.. [nick_concern] Nick's concerns about mutability
|
||
(https://mail.python.org/pipermail/python-dev/2016-June/144883.html)
|
||
|
||
.. [pep422] PEP 422
|
||
(https://www.python.org/dev/peps/pep-0422/#order-preserving-classes)
|
||
|
||
.. [pep487] PEP 487
|
||
(https://www.python.org/dev/peps/pep-0487/#defining-arbitrary-namespaces)
|
||
|
||
.. [orig] original discussion
|
||
(https://mail.python.org/pipermail/python-ideas/2013-February/019690.html)
|
||
|
||
.. [followup1] follow-up 1
|
||
(https://mail.python.org/pipermail/python-dev/2013-June/127103.html)
|
||
|
||
.. [followup2] follow-up 2
|
||
(https://mail.python.org/pipermail/python-dev/2015-May/140137.html)
|
||
|
||
|
||
Copyright
|
||
===========
|
||
This document has been placed in the public domain.
|
||
|
||
|
||
|
||
..
|
||
Local Variables:
|
||
mode: indented-text
|
||
indent-tabs-mode: nil
|
||
sentence-end-double-space: t
|
||
fill-column: 70
|
||
coding: utf-8
|
||
End:
|