Coordinates not quantified / quantifiable

[mueslo]

If I have a DataArray or Dataset, I would expect there to be some way to quantify not just the data, but also the coordinates. <o>.pint.quantify() seems to only quantify the data.

import xarray as xr
import pint
import pint_xarray
ureg = pint.UnitRegistry(force_ndarray_like=True)

ds = xr.Dataset(
    {'z': (("x","y"), np.random.random((2, 3)), {"units": "m"})},
    coords={"x": ("x", [0, 1], {"units": "m"}), 
            "y": ("y", [5, 6, 7], {"units": "m"})})
ds = ds.pint.quantify(unit_registry=ureg)
ds.x + 2*ureg.cm
# DimensionalityError: Cannot convert from 'dimensionless' to 'centimeter'

Manually running quantify on the coordinate gives:

ds.x.pint.quantify()
# ValueError: cannot create a Dataset from a DataArray with the same name as one of its coordinates

Manually constructing from quantified coordinates leads to an explicit dequantify:

x = xr.DataArray([0, 1], attrs={'units': 'm'}, dims=('x',))
x = x.pint.quantify(unit_registry=ureg)

ds = xr.Dataset(
    {'z': (("x","y"), np.random.random((2, 3)), {"units": "m"})},
    coords={"x": x, 
            "y": ("y", [5, 6, 7], {"units": "m"})})
# UnitStrippedWarning: The unit of the quantity is stripped when downcasting to ndarray.
#ds = ds.pint.quantify(unit_registry=ureg)
ds.x + 2*ureg.cm
# DimensionalityError: Cannot convert from 'dimensionless' to 'centimeter'

Is this functionality not expected to work?

[mueslo]

Okay, what I was doing was more like

ds = xr.Dataset(
    {'z': (("x","y"), np.random.random((2, 3)), {"units": "m"})},
    coords={"x": [0, 1], 
            "y": [5, 6, 7]})
ds.x.attrs['units'] = 'm' 
ds = ds.pint.quantify(unit_registry=ureg)

which leads to ds.z being correctly quantified, but not ds.x

Instead running

ds = xr.Dataset(
    {'z': (("x","y"), np.random.random((2, 3)), {"units": "m"})},
    coords={"xx": ("x", [0, 1], {"units": "m"}), 
            "yy": ("y", [5, 6, 7], {"units": "m"})})
ds = ds.pint.quantify(unit_registry=ureg)

Leads to both being correctly quantified. As a beginner with pint/xarray, it's not obvious.

Edit: Okay, but this leads to a Dataset/DataArray without associated indexed dimension coordinates. It seems the index coordinate always drops units. Why?

[mueslo]

https://pint-xarray.readthedocs.io/en/latest/generated/xarray.Dataset.pint.quantify.html#xarray-dataset-pint-quantify

As units in dimension coordinates are not supported until xarray changes the way it implements indexes, these units will be set as attributes.

[keewis]

Note that we're currently actively working on removing that restriction in xarray (will still need some time, though). Once that is done this should just work.

In the meantime, I guess we could make the note more obvious, maybe by not combining it with the one for dask or by moving the dask note to the Notes section and making this one a warning.

[keewis]

reopening, to be closed by #126