Inconsistent behavior with fsspec.open with mode='r' vs. mode='rb'

It's unclear to me how to use fsspec.open as a contextmanager. The behavior of this function seems inconsistent. The inconsistency is somehow related to the mode argument. I will try to illustrate this with an example.

Create a test file

Bypass fsspec completely

fname = 'test.txt'
with open(fname, mode='w') as f:
    f.write('hello')

Open the file with a FileSystem instance

When I open the file via an instantiated FileSystem instance, everything works as I expect

fs = fsspec.implementations.local.LocalFileSystem()
with fs.open(fname, mode='r') as fp:
    print(type(fp))
# -> <class '_io.TextIOWrapper'>
with fs.open(fname, mode='rb') as fp:
    print(type(fp))
# -> <class '_io.BufferedReader'>

The objects yielded to the context manager look like standard open file objects and can be used as such throughout python.

Open the file via fsspec.open

with fsspec.open(fname, mode='r') as fp:
    print(type(fp))
# -> <class '_io.TextIOWrapper'>
with fsspec.open(fname, mode='rb') as fp:
    print(type(fp))
# -> <class 'fsspec.implementations.local.LocalFileOpener'>

With mode='r', the fsspec.open yields object is the same type of object as fs.open. But with mode='rb', we get a LocalFileOpener. ⚠️ this is the key problem. In order to get a BufferedReader, we need an additional context manager!

with fsspec.open(fname) as fp:
    with fp as fp2:
        print(type(fp2))
# -> <class '_io.BufferedReader'>

I can't figure out what this LocalFileOpener object is. It's not documented in the API docs. Most importantly for my purposes, xarray can't figure out what to do with it if I pass it to open_dataset. In contrast, it handles an _io.BufferedReader object fine.

Proposed resolution

I would propose to remove the inconsistency and have with fsspec.open(fname, mode='rb') yield an _io.BufferedReader object. However, I recognize this could be a breaking change for some applications that rely on the LocalFileOpener.

The issue does not name a source file or test path. Reproduce the two fsspec.open examples, then trace how mode='r' and mode='rb' enter their context managers and compare that behavior with FileSystem.open. Done means the intended context-manager behavior is consistent and remains usable by xarray, with regression coverage for both modes.