group_files does not support FSFile objects (TypeError: unhasshable type 'FSFile')

[gerritholl]

Feature Request

Is your feature request related to a problem? Please describe.

When I pass FSFile objects to Multiscene.from_files, satpy.readers.group_files fails with ValueError:

import satpy
from fsspec.implementations.local import LocalFileSystem

fns = ['/media/nas/x21308/abi/ABI-L1B-RadC/2019/023/18/OR_ABI-L1b-RadC-M3C01_G16_s20190231812167_e20190231814540_c20190231814581.nc',
  '/media/nas/x21308/abi/ABI-L1B-RadC/2019/023/18/OR_ABI-L1b-RadC-M3C01_G16_s20190231822167_e20190231824540_c20190231824584.nc',
  '/media/nas/x21308/abi/ABI-L1B-RadC/2019/023/18/OR_ABI-L1b-RadC-M3C01_G16_s20190231832167_e20190231834540_c20190231834583.nc']

lfs = LocalFileSystem()

fsfs = [satpy.readers.FSFile(fn, fs=lfs) for fn in fns]

satpy.readers.group_files(fsfs)

fails with:

Traceback (most recent call last):
  File "/home/gholl/checkouts/protocode/mwe/typeerror-fsfile.py", line 12, in <module>
    satpy.readers.group_files(fsfs)
  File "/data/gholl/miniconda3/envs/py39/lib/python3.9/site-packages/satpy/readers/__init__.py", line 90, in group_files
    reader_files = _assign_files_to_readers(
  File "/data/gholl/miniconda3/envs/py39/lib/python3.9/site-packages/satpy/readers/__init__.py", line 122, in _assign_files_to_readers
    files_to_sort = set(files_to_sort)
TypeError: unhashable type: 'FSFile'

Since group_files is used by MultiScene.from_files, the same exception occurs when trying to pass FSFile objects to Multiscene.from_files.

Describe the solution you'd like

I would like to be able to pass FSFile objects to MultiScene.from_files.

The most reasonable implementation would probably to make FSFile objects hashable. Since both filenames and filesystems are hashable, this should not be a problem.

Describe any changes to existing user workflow

No backward compatibility problems foreseen.

Additional context

I'm not sure if I could solve this differently.