Fix the units and dimensionality properties (#31)

* return None if the wrapped data is not a quantity

* fix the units setattr

* add docstrings to the units and dimensionality properties

* return a dimensionality of None for non-quantities

* fix the magnitude property

* update whats-new.rst

* shorten the property docstrings

* fix the name of the unitless tests

* mention that units can only be set if the data is not a quantity

* reword the docstring dealing with setting the units

* don't assign a name to the constructed quantity

Author: keewis

docs/whats-new.rst

@@ -9,4 +9,7 @@ What's new
 - rewrite :py:meth:`DataArray.pint.quantify`,
   :py:meth:`Dataset.pint.quantify`, :py:meth:`DataArray.pint.dequantify` and
   :py:meth:`Dataset.pint.dequantify` (:pull:`17`)
-- expose :py:func:`pint_xarray.testing.assert_units_equal` as public API (:pull:`24`)
+- expose :py:func:`pint_xarray.testing.assert_units_equal` as public API (:pull:`24`)
+- fix the :py:attr:`DataArray.pint.units`, :py:attr:`DataArray.pint.magnitude`
+  and :py:attr:`DataArray.pint.dimensionality` properties and add docstrings for
+  all three. (:pull:`31`)

pint_xarray/accessors.py

@@ -4,7 +4,7 @@
 import pint
 from pint.quantity import Quantity
 from pint.unit import Unit
-from xarray import DataArray, register_dataarray_accessor, register_dataset_accessor
+from xarray import register_dataarray_accessor, register_dataset_accessor
 
 from . import conversion
 
@@ -230,22 +230,26 @@ def dequantify(self):
 
     @property
     def magnitude(self):
-        return self.da.data.magnitude
+        """the magnitude of the data or the data itself if not a quantity."""
+        data = self.da.data
+        return getattr(data, "magnitude", data)
 
     @property
     def units(self):
-        return self.da.data.units
+        """the units of the data or :py:obj:`None` if not a quantity.
+
+        Setting the units is possible, but only if the data is not already a quantity.
+        """
+        return getattr(self.da.data, "units", None)
 
     @units.setter
     def units(self, units):
-        quantity = conversion.array_attach_units(self.da.data, units)
-        self.da = DataArray(
-            dim=self.da.dims, data=quantity, coords=self.da.coords, attrs=self.da.attrs
-        )
+        self.da.data = conversion.array_attach_units(self.da.data, units)
 
     @property
     def dimensionality(self):
-        return self.da.data.dimensionality
+        """get the dimensionality of the data or :py:obj:`None` if not a quantity."""
+        return getattr(self.da.data, "dimensionality", None)
 
     @property
     def registry(self):

pint_xarray/tests/test_accessors.py

@@ -123,6 +123,37 @@ def test_roundtrip_data(self, example_unitless_da):
         assert_equal(result, orig)
 
 
+class TestPropertiesDataArray:
+    def test_magnitude_getattr(self, example_quantity_da):
+        da = example_quantity_da
+        actual = da.pint.magnitude
+        assert not isinstance(actual, Quantity)
+
+    def test_magnitude_getattr_unitless(self, example_unitless_da):
+        da = example_unitless_da
+        xr.testing.assert_duckarray_equal(da.pint.magnitude, da.data)
+
+    def test_units_getattr(self, example_quantity_da):
+        da = example_quantity_da
+        actual = da.pint.units
+        assert isinstance(actual, Unit)
+        assert actual == unit_registry.m
+
+    def test_units_setattr(self, example_quantity_da):
+        da = example_quantity_da
+        with pytest.raises(ValueError):
+            da.pint.units = "s"
+
+    def test_units_getattr_unitless(self, example_unitless_da):
+        da = example_unitless_da
+        assert da.pint.units is None
+
+    def test_units_setattr_unitless(self, example_unitless_da):
+        da = example_unitless_da
+        da.pint.units = unit_registry.s
+        assert da.pint.units == unit_registry.s
+
+
 @pytest.fixture()
 def example_unitless_ds():
     users = np.linspace(0, 10, 20)