open() overloads mixes typing.IO and io.IOBase hierarchies

[aucampia]

Currently the overloads for the open builtin uses io.IOBase based types for specific cases (specifically io.{TextIOWrapper,FileIO,BufferedRandom,BufferedWriter,BufferedReader}), but for fallbacks it uses typing.IO based types (specifically typing.{BinaryIO,IO[Any]}).

The same is the case for Path.open and Traversable.open.

This would maybe not be a problem if the io.IOBase classes were considered subclasses, subtypes or implementations of typing.IO, but this is not the case. This can maybe be considered a problem on it's own, but it would still seem better to use a consistent approach for typing.

The practical problem that this causes is that functions that should deal with all result from open will have to accept both typing.IO and io.IOBase.

Written for commit 2217ac8, but can also be seen in typeshed included with mypy 0.910.

[aucampia]

Given the response in #6077, I would assume the right approach is to rather move to typing.IO hierarchies then.

[srittau]

All types returned by open() derive from typing.IO (in the stubs, at least). It should be enough to annotate fields using typing.IO[typing.Any]. Are there any examples where this fails (and that are not related to mypy's open plugin)?

[aucampia]

The core of the issue is this: #6061 - but if the solution is to just not use the io.BaseIO type hierarchy for typing, then I guess it is not really a problem.

[aucampia]

The reason why this all came up, I'm trying to type functions that should accept things returned by open, and then would pass them onto TextIOWrapper, and I was typing them using io.IOBase heirarchy, but if I should not do that, then problem solved. I just did not know I should not do that. Apologies for the issue spam. I think the documentation could be clearer, but I understand now.

[aucampia]

Closing this issue as all returned types do derive from typing.IO, and typing.IO should be used instead of io.*IOBase when typing things.

[srittau]

@aucampia I agree. The typing documentation is lacking. I added better documentation for I/O as a todo item for our not-yet-written Best Practices document: python/typing#851.