grid_mapping attrs lead to failure of cf writer

[zxdawn]

Describe the bug

cf writer works well for a DataArray with crs and geotransform written by rioxarray.
But, if we remove these spatial attrs and add the area attrs, cf writer failed.

To Reproduce

import numpy as np
from pyproj import CRS
import xarray as xr
import rioxarray
from satpy import Scene
from affine import Affine
from pyresample.geometry import AreaDefinition


def fake_data():
    test = xr.DataArray(data=np.random.rand(1157, 1235), dims=('y', 'x'))

    test.rio.write_transform(Affine(30.0, 0.0, 262056.03103060447, 0.0, -30.0, 2143592.6343544293), inplace=True)
    test.rio.write_crs(CRS.from_epsg(32643), inplace=True)

    return test

# write data using netcdf directly (version1)
test = fake_data()
test.to_netcdf('test.nc')

# add to Scene (version2)
scn = Scene()
scn['test'] = test
scn.save_datasets(writer="cf", filename='test.nc')

# remove spatial info and write area (version3)
scn = Scene()
# add area
area_def = AreaDefinition('id', 'description', 'proj_id',
                          CRS.from_epsg(32643), 1235, 1157, (-15.0, 1141.0, 1249.0, 15.0))
test.attrs['area'] = area_def
test = test.drop_vars(['spatial_ref'])
# add to Scene
scn['test'] = test
scn.save_datasets(writer="cf", filename='test.nc')

Expected behavior

cf writer works with deleted rioxarray info and manually added area attrs.

Actual results

Error output of version3 in code

File [~/miniconda3/envs/hyperch4/lib/python3.11/site-packages/xarray/coding/variables.py:181](https://file+.vscode-resource.vscode-cdn.net/Users/xinz/Documents/github/HyperCH4/notebooks/~/miniconda3/envs/hyperch4/lib/python3.11/site-packages/xarray/coding/variables.py:181), in safe_setitem(dest, key, value, name)
    179 if key in dest:
    180     var_str = f" on variable {name!r}" if name else ""
--> 181     raise ValueError(
    182         f"failed to prevent overwriting existing key {key} in attrs{var_str}. "
    183         "This is probably an encoding field used by xarray to describe "
    184         "how a variable is serialized. To proceed, remove this key from "
    185         "the variable's attributes manually."
    186     )
    187 dest[key] = value

ValueError: failed to prevent overwriting existing key grid_mapping in attrs. This is probably an encoding field used by xarray to describe how a variable is serialized. To proceed, remove this key from the variable's attributes manually.

Actually, I don't see the grid_mapping attrs in scn['test']:

And, I suppose the output netcdf file from version2 has wrong grid_mapping name:

Environment Info:

• Satpy Version: 0.14.3.dev7585+gf018f1276
• PyResample Version:1.27.1

[zxdawn]

Well. I checked ds['test'] at this line:

satpy/satpy/writers/cf_writer.py

Line 1172 in 34e2bd9

for group_name, ds in grouped_datasets.items():

It does have grid_mapping attrs!

Anyway, this issue can be fixed by manually passing exclude_attrs=['grid_mapping'] to save_datasets.

[zxdawn]

Oh, I mixed rioxarray with pyresample in this case. Because pyresample AreDefinition can handle similar things and is more powerful, we can just use AreaDefinition:

import numpy as np
import xarray as xr
import rioxarray
from satpy import Scene
from pyresample.geometry import AreaDefinition

test = xr.DataArray(data=np.random.rand(1157, 1235), dims=('y', 'x'), name='test')

area_def = AreaDefinition.from_ul_corner(area_id='area_id', projection='EPSG:32643',
                                        shape=(1157, 1235),
                                        upper_left_extent=(262056.03103060447, 2143592.6343544293),
                                        resolution=(30, 30))

test.attrs['area'] = area_def

scn = Scene()
scn['test'] = test
scn.save_datasets(writer="cf", filename='test.nc')

The exported netcdf can be decoded successfully:

ds = xr.open_dataset('test.nc', decode_coords='all')

print(ds.rio.crs)
# CRS.from_epsg(32643)