Zarr backend should avoid checking for invalid encodings

[dcherian]

What is your issue?

The zarr backend has a list of "valid" encodings that needs to be updated any time zarr adds something new (e.g. #6348)

xarray/xarray/backends/zarr.py

Lines 215 to 234 in 53172cb

	valid_encodings = {
	"chunks",
	"compressor",
	"filters",
	"cache_metadata",
	"write_empty_chunks",
	}
	if raise_on_invalid:
	invalid = [k for k in encoding if k not in valid_encodings]
	if invalid:
	raise ValueError(
	f"unexpected encoding parameters for zarr backend: {invalid!r}"
	)
	else:
	for k in list(encoding):
	if k not in valid_encodings:
	del encoding[k]
	chunks = _determine_zarr_chunks(

Can we get rid of this? I don't know the backends code well, but won't all our encoding parameters have been removed by this stage? The raise_on_invalid kwarg suggests so.

@tomwhite points out that zarr will raise a warning:

>>> zarr.create((1,), blah=1)
/Users/tom/miniconda/envs/sgkit-3.8/lib/python3.8/site-packages/zarr/creation.py:221: UserWarning: ignoring keyword argument 'blah'
  warn('ignoring keyword argument %r' % k)
<zarr.core.Array (1,) float64>

[jhamman]

It would be nice to avoid this process but I think we need to keep it. Without extract_zarr_variable_encoding and checks against a valid set of options, we have no way to filter out encoding keys that do not apply to the backend. By way of an example:

In [1]: import xarray as xr
In [2]: ds = xr.tutorial.open_dataset('rasm').chunk('100mb')
In [3]: ds.Tair.encoding
Out[3]:
{'source': '/Users/jhamman/Library/Caches/xarray_tutorial_data/eee06791cc19e59f074155a82a7ffe90-rasm.nc',
 'original_shape': (36, 205, 275),
 'dtype': dtype('float64'),
 '_FillValue': 9.969209968386869e+36,
 'coordinates': 'yc xc'}

Passing most of these to zarr.create would result in a UserWarning. We could inpspect zarr.create to pull the list of valid arguments but I think it is probably cleaner to just list the expected arguments as we do.

[dcherian]

Thanks @jhamman

[tomwhite]

Thanks for confirming @jhamman