[Python-ideas] Adding collections.abc.Ordered

[Guido van Rossum]

Is there also a collections.Reversible? Either way, can you report that bug
in the typehinting tracker on GitHub.

--Guido (mobile)
On Dec 26, 2015 5:01 AM, "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.
>>
>
> Actually we already have such abstract class. It's typing.Reversible.
> Iterating non-ordered collection doesn't make sense.
>
>    >>> issubclass(list, typing.Reversible)
>    True
>    >>> issubclass(collections.deque, typing.Reversible)
>    True
>    >>> issubclass(collections.OrderedDict, typing.Reversible)
>    True
>    >>> issubclass(type(collections.OrderedDict().items()),
> typing.Reversible)
>    True
>    >>> issubclass(dict, typing.Reversible)
>    False
>    >>> issubclass(set, typing.Reversible)
>    False
>    >>> issubclass(frozenset, typing.Reversible)
>    False
>    >>> issubclass(collections.defaultdict, typing.Reversible)
>    False
>    >>> issubclass(type({}.items()), typing.Reversible)
>    False
>
> Unfortunately the test returns False for tuple, str, bytes, bytearray, and
> array:
>
>    >>> issubclass(tuple, typing.Reversible)
>    False
>    >>> issubclass(str, typing.Reversible)
>    False
>    >>> issubclass(bytes, typing.Reversible)
>    False
>    >>> issubclass(bytearray, typing.Reversible)
>    False
>    >>> issubclass(array.array, typing.Reversible)
>    False
>
> This looks as a bug in typing.Reversible.
>
>
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