Add suggestion in ValueError: Cannot parse units for lat/lon

[raybellwaves]

First time dabbling with pint-xarray.

precip in the ERA5 catalog hosted by planet os is in m (total precipitation). Wanted to convert it to mm. I could have simply multiplied it by 1,000 but was a good excuse to try this package.

import pint_xarray
import xarray as xr


ds = xr.open_mfdataset(
    "s3://era5-pds/zarr/2020/01/data/precipitation_amount_1hour_Accumulation.zarr",
    engine="zarr",
)
ds = ds.drop("time1_bounds")

q = ds.pint.quantify(precipitation_amount_1hour_Accumulation="m")

ValueError                                Traceback (most recent call last)
<ipython-input-42-fa56e0f4f598> in <module>
----> 1 q = ds.pint.quantify(precipitation_amount_1hour_Accumulation="m")

~/opt/miniconda3/envs/main/lib/python3.9/site-packages/pint_xarray/accessors.py in quantify(self, units, unit_registry, **unit_kwargs)
    972 
    973         if invalid_units:
--> 974             raise ValueError(format_error_message(invalid_units, "parse"))
    975 
    976         return self.ds.pipe(conversion.strip_unit_attributes).pipe(

ValueError: Cannot parse units:
 -- invalid units for variable 'lat': degrees_north (attribute) (reason: 'degrees_north' is not defined in the unit registry)
 -- invalid units for variable 'lon': degrees_east (attribute) (reason: 'degrees_east' is not defined in the unit registry)

From perusing the issues it's probably related to #26

Tested on main as well

mamba create -n test_env python=3.9 --y
conda activate test_env
mamba install -c conda-forge xarray dask zarr s3fs --y
pip install pip install git+https://github.com/xarray-contrib/pint-xarray.git

[raybellwaves]

The docs help here: https://pint-xarray.readthedocs.io/en/latest/examples/plotting.html#convert-units-into-a-format-understood-by-pint

works fine when doing

ds.lat.attrs["units"] = "degree"
ds.lon.attrs["units"] = "degree"

You could parse for lat/lon in the ValueError: Cannot parse units and explicitly state you could try setting ds.variable.attrs["units"] = "degree" if it contains lat/lon/latitude/longitude etc. or look for degrees_[north/east/south/west].

[keewis]

the code in the tutorial notebook really is a workaround, and I think we'll change that to depend on cf-xarray>0.5.2 (once that is released) which provides a UnitRegistry that can parse these units (and also other cfunits like m s-2). We'll still need to figure out some issues regarding the registering of UnitRegistry objects, though.

Once again, thanks for opening these issues. pint-xarray is currently in a state where most things should work but there are still some issues that need to be fixed (both bugs and design choices), so I really appreciate the feedback.