* collections.abc.Ordered
* collections.abc.Reversible
* collections.abc.Infinite [...]
* collections.abc.Sorted ?
* collections.abc.Recursive ?
Rationale:
These are all attributes of collections that would allow us to reason about [complexity, types, runtime]?
[While someone is at it, annotating functions and methods with complexity class URI fragments accessible at runtime could also be useful for [dynamic programming].]
* Ordered is justified by this thread
* Reversible is distinct from Ordered (because Infinite sequences)
* Infinite is the distinction between Ordered and Reversible
* collections.abc.Sorted would be a useful property to keep track of (because then you don't have to do element-wise comparisons for each collection member)
...
* collections.abc.Recursive could also be a useful property to mixin [again for dynamic programming]
https://en.wikipedia.org/wiki/Dynamic_programming
* collections.abc.Ordered
* collections.abc.Reversible
* collections.abc.Infinite [...]* collections.abc.Sorted ?
On Dec 26, 2015 7:24 PM, "Andrew Barnert via Python-ideas" <python-ideas@python.org> wrote:On Dec 26, 2015, at 16:09, Serhiy Storchaka <storchaka@gmail.com> wrote:
>
>> On 27.12.15 01:05, Guido van Rossum wrote:
>> There is a precedent for declaring that a method isn't implemented:
>> __hash__. The convention is to set it to None in the subclass that
>> explicitly doesn't want to implement it. The __subclasshook__ in
>> collections.Hashable checks for this. The pattern is also used for
>> __await__.
>
> Yes, this was the first thing that I tried, but it doesn't work, as shown in my example in issue25864. This is yet one thing that should be fixed in Reversible.
As Guido pointed out in your typehinting #170, that isn't a bug with typing.Reversible. You can't use typing types in runtime type tests with issubclass. That isn't supposed to work, and the fact that it often kind of does work is actually a bug that he's fixing. So the fact that it doesn't work in this case is correct.
That also means your attempted solution to this thread is wrong; typing.Reversible cannot be used as a substitute for collections.abc.Ordered.
> May be we have to use this idiom more widely, and specially handle assigning special methods to None. The error message "'sometype' can't be reverted" looks better than "'NoneType' is not callable".
I agree with this. A new collections.abc.Reversible (interposed between Iterable and Sequence) would be a potential substitute for Ordered, and would have this problem, which would be solvable by treating __reversed__ = None specially, just like __hash__ = None. And I'm pretty sure it would come up in practice (see issue 25864). And once we've got two or three special methods doing this instead of one, making it more general does sound like a good idea. So, if we need Reversible as a substitute for Ordered, then I think we want the general "is None" test.
But I'm still not sure Reversible is a good substitute for Ordered (again, consider an infinitely long collection, or just a lazy proxy that doesn't compute values until needed and doesn't know its length in advance--they're clearly ordered, and just as clearly not reversible), and I'm not sure we actually need either Reversible or Ordered in the first place.
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