[Python-ideas] Adding collections.abc.Ordered

[Amir Rachum]

I would like to expand on my original use case, which I hope will provide a
more persuading argument.
As part of BasicStruct I intend to allow the use of mapping types as
__slots__, with the semantics of default values.
So it'll look something like this:

    class Point(BasicStruct):
        __slots__ = {'x': 5, 'y': 7}

My fear is that someone will just use a dict literal as stated above and
try to create an object with positional arguments:

    Point(0)

The problem is that iteration order over a dict is not guaranteed, so this
might initialize Point(x=0, y=7) like the use intends, or Point(x=5, y=0).
So in this case I want to disable positional arguments. However, positional
arguments for initializing struct are pretty convenient, so I would like to
make that available as far as possible. An implementation using Ordered
might look like this:

    class BasicStruct(object):
        """Class for holding struct-like objects."""

        __slots__ = ()  # should be extended by deriving classes

        def __init__(self, *args, **kwargs):
            default_values = isinstance(self.__slots__, Mapping)
            ordered = isinstance(self.__slots__, Ordered)

            if args and not ordered:
                raise ValueError("Can't pass non-keyword arguments to {},
since "
                                 "__slots__ was declared with an unordered "

 "iterable.".format(self.__class__.__name__))

            arg_pairs = zip(self.__slots__, args)
            for key, value in chain(arg_pairs, six.iteritems(kwargs)):
                setattr(self, key, value)

            for key in self.__slots__:
                if not hasattr(self, key):
                    default_value = None
                    if default_values:
                        default_value = self.__slots__[key]
                    setattr(self, key, default_value)


This will allow users who are interested in positional argument
initialization to use an Ordered mapping (such as OrderedDict, or a custom
collection).
I would like to emphasize again that the most compelling reason for me that
this should be part of the stdlib is that there is no workaround for this
that also allows users to use ordered custom collections. There is no way
to check for "promised ordering" in any functional manner.
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