[Python-ideas] Deprecating the old-style sequence protocol

[Nick Coghlan]

On 27 December 2015 at 17:30, Serhiy Storchaka <storchaka at gmail.com> wrote:
> On 27.12.15 08:22, Nick Coghlan wrote:
>>
>> These days, without considering the presence or absence of any
>> non-dunder methods, the core distinction between sequences,
>> multi-dimensional arrays and arbitrary mappings really lies in the
>> type signature of the key parameter to__getitem__ et al (assuming a
>> suitably defined Index type hint):
>>
>>      MappingKey = Any
>>      DictKey = collections.abc.Hashable
>>      SequenceKey = Union[Index, slice]
>>      ArrayKey = Union[SequenceKey, Tuple["ArrayKey", ...]]
>
> ArrayKey also includes Ellipsis.

You're right, I was mistakenly thinking that memoryview implemented
tuple indexing without ellipsis support, but it actually doesn't
implement multi-dimensional indexing at all - once you cast to a
multi-dimensional shape, most forms of subscript lookup are no longer
permitted at all by the current implementation. So a more accurate
array key description would look like:

    ArrayKey = Union[SequenceKey, type(Ellipsis), Tuple["ArrayKey", ...]]

(I spelled out Ellipsis to minimise confusion with the
tuple-as-frozen-list typing notation)

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia