[Python-ideas] Adding collections.abc.Ordered

[Neil Girdhar]

FYI: 1. might not work since the elements in a dict may not be sortable.

On Sat, Nov 7, 2015 at 12:06 PM Serhiy Storchaka <storchaka at gmail.com>
wrote:

> On 07.11.15 00:11, Amir Rachum wrote:
> > I am suggesting the addition of a collections abstract base class called
> > "Ordered". Its meaning is that a collection's iteration order is part of
> > its API. The bulk of this mail describes a use case for this. The reason
> > I believe that such abstract base class is required is that there is no
> > way to test this behavior in a given class. An ordered collection has
> > the exact same interface as an unordered collection (e.g, dict and
> > OrderedDict), other than a _promise_ of the API that the order in which
> > this collection will be iterated has some sort of meaning (In
> > OrderedDict, it is the order in which keys were added to it.)
> >
> >
> > As examples, set, frozenset, dict and defaultdict should *not* be
> > considered as ordered. list, OrderedDict, deque and tuple should be
> > considered ordered.
>
> I just wanted to offer this idea. I have two use cases:
>
> 1. Human-readable output. If the collection is not ordered, it is
> preferable to sort it before output. pprint and testing frameworks will
> benefit from this.
>
> 2. Serialization. If the mapping is not ordered, we can serialize its
> content as dict (that can be more efficient), otherwise we have to
> serialize it as a sequence of key-value pairs.
>
> Actually we need only two functions: test if the collection is ordered,
> and a way to register the class as ordered (or unordered).
>
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