use .pint.quantify() as a identity operator

docs/whats-new.rst

@@ -12,6 +12,9 @@ What's new
   By `Justus Magin <https://github.com/keewis>`_.
 - fix "quantifying" dimension coordinates (:issue:`105`, :pull:`174`).
   By `Justus Magin <https://github.com/keewis>`_.
+- allow using :py:meth:`DataArray.pint.quantify` and :py:meth:`Dataset.pint.quantify`
+  as identity operators (:issue:`47`, :pull:`175`).
+  By `Justus Magin <https://github.com/keewis>`_.
 
 0.2.1 (26 Jul 2021)
 -------------------

pint_xarray/accessors.py

@@ -2,7 +2,7 @@
 import itertools
 
 import pint
-from pint import Quantity, Unit
+from pint import Unit
 from xarray import register_dataarray_accessor, register_dataset_accessor
 from xarray.core.dtypes import NA
 
@@ -71,16 +71,6 @@ def zip_mappings(*mappings, fill_value=None):
     return zipped
 
 
-def merge_mappings(first, *mappings):
-    result = first.copy()
-    for mapping in mappings:
-        result.update(
-            {key: value for key, value in mapping.items() if value is not None}
-        )
-
-    return result
-
-
 def units_to_str_or_none(mapping, unit_format):
     formatter = str if not unit_format else lambda v: unit_format.format(v)
 
@@ -109,8 +99,8 @@ def either_dict_or_kwargs(positional, keywords, method_name):
 
 
 def get_registry(unit_registry, new_units, existing_units):
-    units = merge_mappings(existing_units, new_units)
-    registries = {unit._REGISTRY for unit in units.values() if isinstance(unit, Unit)}
+    units = itertools.chain(new_units.values(), existing_units.values())
+    registries = {unit._REGISTRY for unit in units if isinstance(unit, Unit)}
 
     if unit_registry is None:
         if not registries:
@@ -133,7 +123,7 @@ def get_registry(unit_registry, new_units, existing_units):
 
 
 def _decide_units(units, registry, unit_attribute):
-    if units is _default and unit_attribute is _default:
+    if units is _default and unit_attribute in (None, _default):
         # or warn and return None?
         raise ValueError("no units given")
     elif units in no_unit_values or isinstance(units, Unit):
@@ -321,13 +311,6 @@ def quantify(self, units=_default, unit_registry=None, **unit_kwargs):
         array([0.4, 0.9])
         Dimensions without coordinates: wavelength
         """
-
-        if isinstance(self.da.data, Quantity):
-            raise ValueError(
-                f"Cannot attach unit {units} to quantity: data "
-                f"already has units {self.da.data.units}"
-            )
-
         if units is None or isinstance(units, (str, pint.Unit)):
             if self.da.name in unit_kwargs:
                 raise ValueError(
@@ -347,11 +330,11 @@ def quantify(self, units=_default, unit_registry=None, **unit_kwargs):
         new_units = {}
         invalid_units = {}
         for name, (unit, attr) in possible_new_units.items():
-            if unit is not _default or attr is not _default:
+            if unit not in (_default, None) or attr not in (_default, None):
                 try:
                     new_units[name] = _decide_units(unit, registry, attr)
                 except (ValueError, pint.UndefinedUnitError) as e:
-                    if unit is not _default:
+                    if unit not in (_default, None):
                         type = "parameter"
                         reported_unit = unit
                     else:
@@ -373,7 +356,7 @@ def quantify(self, units=_default, unit_registry=None, **unit_kwargs):
             for name, (old, new) in zip_mappings(
                 existing_units, new_units, fill_value=_default
             ).items()
-            if old is not _default and new is not _default
+            if old is not _default and new is not _default and old != new
         }
         if overwritten_units:
             errors = {
@@ -1062,7 +1045,7 @@ def quantify(self, units=_default, unit_registry=None, **unit_kwargs):
         new_units = {}
         invalid_units = {}
         for name, (unit, attr) in possible_new_units.items():
-            if unit is not _default or attr is not _default:
+            if unit is not _default or attr not in (None, _default):
                 try:
                     new_units[name] = _decide_units(unit, registry, attr)
                 except (ValueError, pint.UndefinedUnitError) as e:
@@ -1088,7 +1071,7 @@ def quantify(self, units=_default, unit_registry=None, **unit_kwargs):
             for name, (old, new) in zip_mappings(
                 existing_units, new_units, fill_value=_default
             ).items()
-            if old is not _default and new is not _default
+            if old is not _default and new is not _default and old != new
         }
         if overwritten_units:
             errors = {

pint_xarray/conversion.py

@@ -33,6 +33,9 @@ def array_attach_units(data, unit):
         raise ValueError(f"cannot use {unit!r} as a unit")
 
     if isinstance(data, pint.Quantity):
+        if data.units == unit:
+            return data
+
         raise ValueError(
             f"Cannot attach unit {unit!r} to quantity: data "
             f"already has units {data.units}"

pint_xarray/tests/test_accessors.py

@@ -11,7 +11,6 @@
     assert_equal,
     assert_identical,
     assert_units_equal,
-    raises_regex,
     requires_bottleneck,
     requires_dask_array,
     requires_scipy,
@@ -111,10 +110,62 @@ def test_override_units(self, example_unitless_da, no_unit_value):
         with pytest.raises(AttributeError):
             result["u"].data.units
 
-    def test_error_when_already_units(self, example_quantity_da):
+    def test_error_when_changing_units(self, example_quantity_da):
         da = example_quantity_da
-        with raises_regex(ValueError, "already has units"):
-            da.pint.quantify()
+        with pytest.raises(ValueError, match="already has units"):
+            da.pint.quantify("s")
+
+    def test_attach_no_units(self):
+        arr = xr.DataArray([1, 2, 3], dims="x")
+        quantified = arr.pint.quantify()
+        assert_identical(quantified, arr)
+        assert_units_equal(quantified, arr)
+
+    def test_attach_no_new_units(self):
+        da = xr.DataArray(unit_registry.Quantity([1, 2, 3], "m"), dims="x")
+        quantified = da.pint.quantify()
+        assert_identical(quantified, da)
+        assert_units_equal(quantified, da)
+
+    def test_attach_same_units(self):
+        da = xr.DataArray(unit_registry.Quantity([1, 2, 3], "m"), dims="x")
+        quantified = da.pint.quantify("m")
+        assert_identical(quantified, da)
+        assert_units_equal(quantified, da)
+
+    def test_error_when_changing_units_dimension_coordinates(self):
+        arr = xr.DataArray(
+            [1, 2, 3],
+            dims="x",
+            coords={"x": ("x", [-1, 0, 1], {"units": unit_registry.Unit("m")})},
+        )
+        with pytest.raises(ValueError, match="already has units"):
+            arr.pint.quantify({"x": "s"})
+
+    def test_dimension_coordinate_array(self):
+        ds = xr.Dataset(coords={"x": ("x", [10], {"units": "m"})})
+        arr = ds.x
+
+        # does not actually quantify because `arr` wraps a IndexVariable
+        # but we still get a `Unit` in the attrs
+        q = arr.pint.quantify()
+        assert isinstance(q.attrs["units"], Unit)
+
+    def test_dimension_coordinate_array_already_quantified(self):
+        ds = xr.Dataset(coords={"x": ("x", [10], {"units": unit_registry.Unit("m")})})
+        arr = ds.x
+
+        with pytest.raises(ValueError):
+            arr.pint.quantify({"x": "s"})
+
+    def test_dimension_coordinate_array_already_quantified_same_units(self):
+        ds = xr.Dataset(coords={"x": ("x", [10], {"units": unit_registry.Unit("m")})})
+        arr = ds.x
+
+        quantified = arr.pint.quantify({"x": "m"})
+
+        assert_identical(quantified, arr)
+        assert_units_equal(quantified, arr)
 
     def test_error_on_nonsense_units(self, example_unitless_da):
         da = example_unitless_da
@@ -135,22 +186,6 @@ def test_parse_integer_inverse(self):
         result = da.pint.quantify()
         assert result.pint.units == Unit("1 / meter")
 
-    def test_dimension_coordinate(self):
-        ds = xr.Dataset(coords={"x": ("x", [10], {"units": "m"})})
-        arr = ds.x
-
-        # does not actually quantify because `arr` wraps a IndexVariable
-        # but we still get a `Unit` in the attrs
-        q = arr.pint.quantify()
-        assert isinstance(q.attrs["units"], Unit)
-
-    def test_dimension_coordinate_already_quantified(self):
-        ds = xr.Dataset(coords={"x": ("x", [10], {"units": unit_registry.Unit("m")})})
-        arr = ds.x
-
-        with pytest.raises(ValueError):
-            arr.pint.quantify({"x": "s"})
-
 
 @pytest.mark.parametrize("formatter", ("", "P", "C"))
 @pytest.mark.parametrize("modifier", ("", "~"))
@@ -308,8 +343,35 @@ def test_override_units(self, example_unitless_ds, no_unit_value):
         )
 
     def test_error_when_already_units(self, example_quantity_ds):
-        with raises_regex(ValueError, "already has units"):
-            example_quantity_ds.pint.quantify({"funds": "pounds"})
+        with pytest.raises(ValueError, match="already has units"):
+            example_quantity_ds.pint.quantify({"funds": "kg"})
+
+    def test_attach_no_units(self):
+        ds = xr.Dataset({"a": ("x", [1, 2, 3])})
+        quantified = ds.pint.quantify()
+        assert_identical(quantified, ds)
+        assert_units_equal(quantified, ds)
+
+    def test_attach_no_new_units(self):
+        ds = xr.Dataset({"a": ("x", unit_registry.Quantity([1, 2, 3], "m"))})
+        quantified = ds.pint.quantify()
+
+        assert_identical(quantified, ds)
+        assert_units_equal(quantified, ds)
+
+    def test_attach_same_units(self):
+        ds = xr.Dataset({"a": ("x", unit_registry.Quantity([1, 2, 3], "m"))})
+        quantified = ds.pint.quantify({"a": "m"})
+
+        assert_identical(quantified, ds)
+        assert_units_equal(quantified, ds)
+
+    def test_error_when_changing_units_dimension_coordinates(self):
+        ds = xr.Dataset(
+            coords={"x": ("x", [-1, 0, 1], {"units": unit_registry.Unit("m")})},
+        )
+        with pytest.raises(ValueError, match="already has units"):
+            ds.pint.quantify({"x": "s"})
 
     def test_error_on_nonsense_units(self, example_unitless_ds):
         ds = example_unitless_ds

pint_xarray/tests/test_conversion.py

@@ -78,6 +78,20 @@ class TestArrayFunctions:
                 "already has units",
                 id="unit object on quantity",
             ),
+            pytest.param(
+                Unit("m"),
+                Quantity(np.array([0, 1]), "m"),
+                Quantity(np.array([0, 1]), "m"),
+                None,
+                id="unit object on quantity with same unit",
+            ),
+            pytest.param(
+                Unit("mm"),
+                Quantity(np.array([0, 1]), "m"),
+                None,
+                "already has units",
+                id="unit object on quantity with similar unit",
+            ),
         ),
     )
     def test_array_attach_units(self, data, unit, expected, match):