BUG: Fix interpolate_to_slice unit handling (Fixes Unidata#3341)

The fixup in Unidata#3255 inadvertently added unit parsing to all code paths in
interpolate_to_slice. This breaks any usages that passed in data with
unknown units. Since interpolate_to_slice has no need to do any unit
handling, avoid this unless we're actually given a quantity.

src/metpy/interpolate/slices.py

@@ -7,6 +7,7 @@
 import xarray as xr
 
 from ..package_tools import Exporter
+from ..units import is_quantity, units
 from ..xarray import check_axis
 
 exporter = Exporter(globals())
@@ -49,14 +50,15 @@ def interpolate_to_slice(data, points, interp_type='linear'):
                          'your data has been parsed by MetPy with proper x and y '
                          'dimension coordinates.') from None
 
+    need_quantify = is_quantity(data.data)
     data = data.metpy.dequantify()
     data_sliced = data.interp({
         x.name: xr.DataArray(points[:, 0], dims='index', attrs=x.attrs),
         y.name: xr.DataArray(points[:, 1], dims='index', attrs=y.attrs)
     }, method=interp_type)
     data_sliced.coords['index'] = range(len(points))
 
-    return data_sliced.metpy.quantify()
+    return data_sliced.metpy.quantify() if need_quantify else data_sliced
 
 
 @exporter.export

tests/interpolate/test_slices.py

@@ -95,16 +95,24 @@ def test_ds_xy():
     return ds.metpy.parse_cf()
 
 
-def test_interpolate_to_slice_against_selection(test_ds_lonlat):
+@pytest.mark.parametrize('bad_units', [False, True])
+def test_interpolate_to_slice_against_selection(test_ds_lonlat, bad_units):
     """Test interpolate_to_slice on a simple operation."""
     data = test_ds_lonlat['temperature']
+
+    # interpolate_to_slice shouldn't care about units
+    if bad_units:
+        # Needed so we can go back to using attribute metadata
+        data = data.metpy.dequantify()
+        data.attrs['units'] = 'my_bad_units'
+
     path = np.array([[265.0, 30.],
                      [265.0, 36.],
                      [265.0, 42.]])
     test_slice = interpolate_to_slice(data, path)
     true_slice = data.sel({'lat': [30., 36., 42.], 'lon': 265.0})
     # Coordinates differ, so just compare the data
-    assert_array_almost_equal(true_slice.metpy.unit_array, test_slice.metpy.unit_array, 5)
+    assert_array_almost_equal(true_slice.data, test_slice.data, 5)
 
 
 @needs_cartopy