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[Python-ideas] Adding collections.abc.Ordered

Neil Girdhar mistersheik at gmail.com
Sun Nov 8 14:10:01 EST 2015 
On Sun, Nov 8, 2015 at 2:06 PM Amir Rachum <amir at rachum.com> wrote:

> 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.
>

Did you know that you can register your custom collection with an abc?

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