On 06/07/2016 05:50 PM, Eric Snow wrote:
Overall +1. Some nits below.
Specification
=============
3. types for which `__prepare__()`` returned something other than
``OrderedDict`` (or a subclass) have their ``__definition_order__``
set to ``None``
(unless ``__definition_order__`` is present in
the class dict either by virtue of being in the class body or
because the metaclass inserted it before calling
``type.__new__``)
__definition_order__ = tuple(k for k in locals()
if (!k.startswith('__') or
!k.endswith('__')))
Still mixing C and Python! ;)
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.
If __definition_order__ is supposed to be immutable as well as read-only
then we should convert non-tuples to tuples. No point in letting that
user bug slip through.
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 early everyone they would only
s/early/nearly
Why is __definition_order__ even necessary?
-------------------------------------------
Since the definition order is not preserved in ``__dict__``, it would be
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.
s/would be/is
--
~Ethan~
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