Hi all, Following discussion a few years back (and rough approval from Guido [1]), I started work on using OrderedDict for the class definition namespace by default. The bulk of the effort lay in implementing OrderedDict in C, which I got landed just in time for 3.5. The remaining work was quite minimal and the actual change is quite small. My intention was to land the patch soon, having gone through code review during PyCon. However, Nick pointed out to me the benefit of having a concrete point of reference for the change, as well as making sure it isn't a problem for other implementations. So in that spirit, here's a PEP for the change. Feedback is welcome, particularly from from other implementors. -eric [1] https://mail.python.org/pipermail/python-ideas/2013-February/019704.html ================================================== PEP: XXX Title: Ordered Class Definition Namespace Version: $Revision$ Last-Modified: $Date$ Author: Eric Snow <ericsnowcurrently@gmail.com> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 4-Jun-2016 Python-Version: 3.6 Post-History: 7-Jun-2016 Abstract ======== This PEP changes the default class definition namespace to ``OrderedDict``. Furthermore, the order in which the attributes are defined in each class body will now be preserved in ``type.__definition_order__``. This allows introspection of the original definition order, e.g. by class decorators. Note: just to be clear, this PEP is *not* about changing ``type.__dict__`` to ``OrderedDict``. Motivation ========== Currently 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, if you needed your class definition namespace to be ``OrderedDict`` you had to use 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. 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``. The usefulness of ``OrderedDict``-by-default is greatly increased if the definition order is directly introspectable on classes afterward, particularly by code that is independent of the original class definition. One of the original motivating use cases for this PEP is generic class decorators that make use of the definition order. Changing the default class definition namespace has been discussed a number of times, including on the mailing lists and in PEP 422 and PEP 487 (see the References section below). Specification ============= * the default class *definition* namespace is now ``OrderdDict`` * 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 * ``__definition_order__`` is a read-only attribute * ``__definition_order__`` is always set: * if ``__definition_order__`` is defined in the class body then it is used * types that do not have a class definition (e.g. builtins) have their ``__definition_order__`` set to ``None`` * types for which `__prepare__()`` returned something other than ``OrderedDict`` (or a subclass) have their ``__definition_order__`` set to ``None`` The following code demonstrates roughly equivalent semantics:: class Meta(type): def __prepare__(cls, *args, **kwargs): return OrderedDict() class Spam(metaclass=Meta): ham = None eggs = 5 __definition_order__ = tuple(k for k in locals() if (!k.startswith('__') or !k.endswith('__'))) Note that [pep487_] proposes a similar solution, albeit as part of a broader proposal. Compatibility ============= This PEP does not break backward compatibility, except in the case that someone relies *strictly* on dicts as the class definition namespace. This shouldn't be a problem. 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 ============ type.__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 mostly provide the same semantics. However, using ``OrderedDict`` for ``type,__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. References ========== .. [impl] issue #24254 (https://bugs.python.org/issue24254) .. [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.