issues

Hi,

May I suggest having a default_fillvals option to xarray.open_dataset (and xarray.open_dataarray)?

My problem:

I have netcdf data containing flagged data, that is flagged with the netcdf default fill value of 9.96...e+36. But xarray (0.10.8) only masks arrays that have an explicit fill_value set:

```python import netCDF4, xarray, numpy

nc = netCDF4.Dataset('test.nc', 'w', format='NETCDF4') nc.createDimension('x', 3)

var1 = nc.createVariable('var1', 'f8', ('x',)) var2 = nc.createVariable('var2', 'f8', ('x',), fill_value=netCDF4.default_fillvals['f8'])

var1[:] = numpy.array([0., 1., netCDF4.default_fillvals['f8']]) var2[:] = numpy.array([0., 1., netCDF4.default_fillvals['f8']]) print('netCDF4 var1', nc.variables['var1'][:]) print('netCDF4 var2', nc.variables['var2'][:]) nc.close()

ds = xarray.open_dataset('test.nc') print('xarray var1', ds.var1[:]) print('xarray var2', ds.var2[:]) ```

The problem is, that ds.var1 and ds.var2 are interpreted differently, although netCDF4 shows both as masked: netCDF4 var1 [0.0 1.0 --] netCDF4 var2 [0.0 1.0 --] xarray var1 <xarray.DataArray 'var1' (x: 3)> array([0.00000e+00, 1.00000e+00, 9.96921e+36]) Dimensions without coordinates: x xarray var2 <xarray.DataArray 'var2' (x: 3)> array([ 0., 1., nan]) Dimensions without coordinates: x

I agree, that it is a good default, to mask data, only if the fill_value attribute is set. But I think it would be useful to be able to pass default_fill values to open_dataset to enable reading data, that uses the implicit default values.

What do you think?

Dea all,

I encountered a small unforeseen bug using the decode_cf of open_dataset. By only reading the doc string, I was not aware of the complete meaning of this parameter. Especially, that it overwrites other options.

Example: ds = xarray.open_dataset(file_name, mask_and_scale=True, decode_cf=False)

The result is an unmasked and not sclaed dataset. decode_cf simply overwrites other options. See the code: https://github.com/pydata/xarray/blob/stable/xarray/backends/api.py#L308-L312

A simple solution would be to explain, that decode_cf sets mask_and_scale, decode_times, concat_characters, and decode_coords to False. But probably it would be more convenient to detect option conflicts like in my example and raise a ValueError.

What do you think? Do you prefer any of these options? I could write the little PR.