parse_units instead of parse_expression (#41)

* test for interpreting inverse integer unit

* replace parse_expression with parse_units

* black formatting

* updated what's new

* moved test

* black reformatting

Co-authored-by: keewis <[email protected]>

* rerun black

Co-authored-by: keewis <[email protected]>
Co-authored-by: Keewis <[email protected]>

Author: TomNicholas

docs/whats-new.rst

@@ -3,7 +3,8 @@ What's new
 
 0.2 (*unreleased*)
 ------------------
-
+- rewrite :py:meth:`Dataset.pint.quantify` and :py:meth:`DataArray.pint.quantify`, 
+  to use pint's `parse_units` instead of `parse_expression` (:pull:`40`)
 
 v0.1 (October 26 2020)
 ----------------------

pint_xarray/accessors.py

@@ -132,7 +132,7 @@ def _decide_units(units, registry, unit_attribute):
         raise ValueError("no units given")
     elif units is None:
         # TODO option to read and decode units according to CF conventions (see MetPy)?
-        units = registry.parse_expression(unit_attribute).units
+        units = registry.parse_units(unit_attribute)
     elif isinstance(units, Unit):
         # TODO do we have to check what happens if someone passes a Unit instance
         # without creating a unit registry?
@@ -227,11 +227,7 @@ def quantify(self, units=None, unit_registry=None, **unit_kwargs):
 
         units = either_dict_or_kwargs(units, unit_kwargs, ".quantify")
 
-        registry = get_registry(
-            unit_registry,
-            units,
-            conversion.extract_units(self.da),
-        )
+        registry = get_registry(unit_registry, units, conversion.extract_units(self.da))
 
         unit_attrs = conversion.extract_unit_attributes(self.da)
         new_obj = conversion.strip_unit_attributes(self.da)
@@ -266,8 +262,7 @@ def dequantify(self):
 
         units = units_to_str_or_none(conversion.extract_units(self.da))
         new_obj = conversion.attach_unit_attributes(
-            conversion.strip_units(self.da),
-            units,
+            conversion.strip_units(self.da), units
         )
 
         return new_obj
@@ -498,11 +493,7 @@ def quantify(self, units=None, unit_registry=None, **unit_kwargs):
             b        (x) int64 <Quantity([ 5 -2  1], 'decimeter')>
         """
         units = either_dict_or_kwargs(units, unit_kwargs, ".quantify")
-        registry = get_registry(
-            unit_registry,
-            units,
-            conversion.extract_units(self.ds),
-        )
+        registry = get_registry(unit_registry, units, conversion.extract_units(self.ds))
 
         unit_attrs = conversion.extract_unit_attributes(self.ds)
         new_obj = conversion.strip_unit_attributes(self.ds)

pint_xarray/tests/test_accessors.py

@@ -94,6 +94,12 @@ def test_error_on_nonsense_units(self, example_unitless_da):
         with pytest.raises(UndefinedUnitError):
             da.pint.quantify(units="aecjhbav")
 
+    def test_parse_integer_inverse(self):
+        # Regression test for issue #40
+        da = xr.DataArray([10], attrs={"units": "m^-1"})
+        result = da.pint.quantify()
+        assert result.pint.units == Unit("1 / meter")
+
 
 class TestDequantifyDataArray:
     def test_strip_units(self, example_quantity_da):

pint_xarray/tests/test_conversion.py

@@ -196,11 +196,7 @@ def test_attach_units(self, obj, units):
                 {"a": "K", "b": "hPa", "u": "m"},
                 id="Dataset",
             ),
-            pytest.param(
-                Variable("x", []),
-                {None: "hPa"},
-                id="Variable",
-            ),
+            pytest.param(Variable("x", []), {None: "hPa"}, id="Variable"),
         ),
     )
     def test_attach_unit_attributes(self, obj, units):
@@ -370,9 +366,7 @@ def test_extract_units(self, typename, units):
                 id="Dataset",
             ),
             pytest.param(
-                Variable("x", [], {"units": "hPa"}),
-                {None: "hPa"},
-                id="Variable",
+                Variable("x", [], {"units": "hPa"}), {None: "hPa"}, id="Variable"
             ),
         ),
     )
@@ -446,9 +440,7 @@ def test_strip_units(self, obj):
                 id="Dataset",
             ),
             pytest.param(
-                Variable("x", [], {"units": "hPa"}),
-                {None: "hPa"},
-                id="Variable",
+                Variable("x", [], {"units": "hPa"}), {None: "hPa"}, id="Variable"
             ),
         ),
     )