Some issues with type annotations

Some issues with type annotations that were brought up by @Bvb93 at https://github.com/numpy/numpy/pull/18585

• For asarray, it isn’t clear how to define NestedSequence (typing annotations don’t support recursive types).
• asarray needs to also include array as a valid type for obj (array should not be considred a subtype of NestedSequence)
• The pipe operator for types isn’t valid yet (https://www.python.org/dev/peps/pep-0604/) (it’s used in the NestedSequence type in asarray)
• For stack and concat, the type for arrays should be Tuple[array, ...] instead of Tuple[array]
• For stack and concat (and possibly others), Tuple is too restrictive vs. Sequence.
• unique can be implemented as an @overload (see https://github.com/numpy/numpy/pull/18585#discussion_r591757472). I don’t know if this needs to be changed in the spec.
• The return type for finfo as finfo doesn’t make sense if finfo is a function (see https://github.com/numpy/numpy/pull/18585#discussion_r591763368).
• __getitem__ and __setitem__ should include the ellipsis type (https://github.com/numpy/numpy/pull/18585#discussion_r592256384)
• __len__ should return int (update: __len__ is slated for removal from the spec; see https://github.com/data-apis/array-api/pull/289)
• The return type for shape is currently Union[ Tuple[ int, …], <shape> ], which needs to be addressed (there is a TODO in the spec for this). (update: see https://github.com/data-apis/array-api/pull/289)
• Some of the other types for properties also use <...> types. We should make all the type annotations be valid Python/mypy that can be used in the annotations in the signature.
• Should the array object subclass from typing.Protocol (https://github.com/numpy/numpy/pull/18585#discussion_r592261549)? (I don’t know if this is relevant for the spec).
• The dunder methods (__add__, and so on) should include int, float, (and bool as appropriate) as allowed types for other.

[rgommers]

For stack and concat (and possibly others), Tuple is too restrictive vs. Sequence.

I’m actually not sure Sequence is a good idea. It can be quite hard to deal with this correctly in both Python and C/C++, see e.g. https://stackoverflow.com/questions/43566044/what-is-pythons-sequence-protocol. I remember a lot of cases where we special-cased tuple and list inputs, and other kinds of sequences were doing the wrong thing.

I can be convinced that accepting Union[Tuple[<array>, ...], List[<array>]] is a good idea, but I don’t think Sequence is.

[kgryte]

As all items in this tracking issue have been addressed, will close. Any further issues can be discussed in a new issue and resolved in follow-up PRs.