Merge pull request #25 from openscm/n2o-clarity

Remove N2O ambiguity

CHANGELOG.rst

@@ -3,6 +3,8 @@ Changelog
 
 master
 ------
+
+- (`#25 <https://github.com/openscm/openscm-units/pull/25>`_) Add "N2O_conversions" context to remove ambiguity in N2O conversions
 - (`#23 <https://github.com/openscm/openscm-units/pull/23>`_) Add AR5 GWPs with climate-carbon cycle feedbacks (closes `#22 <https://github.com/openscm/openscm-units/issues/22>`_)
 - (`#20 <https://github.com/openscm/openscm-units/pull/20>`_) Make ``openscm_units.data`` a module by adding an ``__init__.py`` file to it and add docs for ``openscm_units.data`` (closes `#19 <https://github.com/openscm/openscm-units/issues/19>`_)
 - (`#18 <https://github.com/openscm/openscm-units/pull/18>`_) Made NH3 a separate dimension to avoid accidental conversion to CO2 in GWP contexts. Also added an ``nh3_conversions`` context to convert to nitrogen (closes `#12 <https://github.com/openscm/openscm-units/issues/12>`_)

src/openscm_units/_unit_registry.py

@@ -34,9 +34,8 @@
 
 - fairly obvious ones e.g. carbon dioxide emissions can be provided in 'C' or 'CO2' and
   converting between the two is possible
-- less obvious ones e.g. nitrous oxide emissions can be provided in 'N', 'N2O' or
-  'N2ON' (a short-hand which indicates that only the mass of the nitrogen is being counted),
-  we provide conversions between these three
+- less obvious ones e.g. NOx emissions can be provided in 'N' or 'NOx' (a short-hand
+  which is assumed to be NO2), we provide conversions between these two
 - case-sensitivity. In order to provide a simplified interface, using all uppercase
   versions of any unit is also valid e.g. ``unit_registry("HFC4310mee")`` is the same as
   ``unit_registry("HFC4310MEE")``
@@ -85,6 +84,29 @@
     ...     unit_registry("CH4").to("CO2")
     <Quantity(2.75, 'CO2')>
 
+*N2O*
+
+Nitrous oxide emissions are typically reported with units of 'N2O'. However,
+they are also reported with units of 'N2ON' (a short-hand which indicates that
+only the mass of the nitrogen is being counted). Reporting nitrous oxide
+emissions with units of simply 'N' is ambiguous (do you mean the mass of
+nitrogen, so 1 N = 28 / 44 N2O or just the mass of a single N atom, so
+1 N = 14 / 44 N2O). By default, converting 'N2O' <--> 'N' is forbidden to
+prevent this ambiguity. However, the conversion can be performed within the
+context 'N2O_conversions', in which case it is assumed that 'N' just means a
+single N atom i.e. 1 N = 14 / 44 N2O, as shown below:
+
+.. code:: python
+
+    >>> from openscm_units import unit_registry
+    >>> unit_registry("N2O").to("N")
+    pint.errors.DimensionalityError: Cannot convert from 'N2O' ([nitrous_oxide]) to 'N' ([nitrogen])
+
+    # with a context, the conversion becomes legal again
+    >>> with unit_registry.context("N2O_conversions"):
+    ...     unit_registry("N2O").to("N")
+    <Quantity(0.318181818, 'N')>
+
 *NOx*
 
 Like for methane, NOx emissions also suffer from a namespace collision. In order to
@@ -102,11 +124,6 @@
     ...     unit_registry("NOx").to("N")
     <Quantity(0.30434782608695654, 'N')>
 
-    # as an unavoidable side effect, this also becomes possible
-    >>> with unit_registry.context("NOx_conversions"):
-    ...     unit_registry("NOx").to("N2O")
-    <Quantity(0.9565217391304348, 'N2O')>
-
 *NH3*
 
 In order to prevent inadvertent conversions from 'NH3' to 'CO2', the conversion
@@ -144,9 +161,9 @@
     "CO2": ["12/44 * C", "carbon_dioxide"],
     "CH4": "methane",
     "HC50": ["CH4"],
+    "N2O": "nitrous_oxide",
+    "N2ON": ["44/28 * N2O", "nitrous_oxide_farming_style"],
     "N": "nitrogen",
-    "N2O": ["14/44 * N", "nitrous_oxide"],
-    "N2ON": ["14/28 * N", "nitrous_oxide_farming_style"],
     "NO2": ["14/46 * N", "nitrogen_dioxide"],
     # aerosol precursors
     "NOx": "NOx",
@@ -379,16 +396,27 @@ def _load_contexts(self):
         )
         self.add_context(_ch4_context)
 
-        _n2o_context = pint.Context("NOx_conversions")
+        _n2o_context = pint.Context("N2O_conversions")
         _n2o_context = self._add_transformations_to_context(
             _n2o_context,
+            "[nitrous_oxide]",
+            self.nitrous_oxide,
+            "[nitrogen]",
+            self.nitrogen,
+            14 / 44,
+        )
+        self.add_context(_n2o_context)
+
+        _nox_context = pint.Context("NOx_conversions")
+        _nox_context = self._add_transformations_to_context(
+            _nox_context,
             "[nitrogen]",
             self.nitrogen,
             "[NOx]",
             self.NOx,
             (14 + 2 * 16) / 14,
         )
-        self.add_context(_n2o_context)
+        self.add_context(_nox_context)
 
         _nh3_context = pint.Context("NH3_conversions")
         _nh3_context = self._add_transformations_to_context(

tests/unit/test_units.py

@@ -24,9 +24,16 @@ def test_base_unit():
 
 def test_nitrogen():
     N = unit_registry("N")
-    np.testing.assert_allclose(N.to("N2ON").magnitude, 28 / 14)
     np.testing.assert_allclose(N.to("NO2").magnitude, 46 / 14)
 
+    # can only convert to N with right context
+    with pytest.raises(DimensionalityError):
+        N.to("N2ON")
+
+    with unit_registry.context("N2O_conversions"):
+        np.testing.assert_allclose(N.to("N2ON").magnitude, 28 / 14)
+        np.testing.assert_allclose(N.to("N2O").magnitude, 44 / 14)
+
 
 def test_nox():
     NOx = unit_registry("NOx")
@@ -42,8 +49,8 @@ def test_nox():
         np.testing.assert_allclose(N.to("NOx").magnitude, 46 / 14)
         np.testing.assert_allclose(NO2.to("NOx").magnitude, 1)
         np.testing.assert_allclose(NOx.to("NO2").magnitude, 1)
-        # this also becomes allowed, unfortunately...
-        np.testing.assert_allclose(NOx.to("N2O").magnitude, 44 / 46)
+        with pytest.raises(DimensionalityError):
+            NOx.to("N2O")
 
 
 def test_ammonia():
@@ -149,32 +156,36 @@ def test_a():
 
 def test_context():
     CO2 = unit_registry("CO2")
-    N = unit_registry("N")
+    N2ON = unit_registry("N2ON")
+    N2O = unit_registry("N2O")
     with unit_registry.context("AR4GWP100"):
-        np.testing.assert_allclose(CO2.to("N").magnitude, 14 / (44 * 298))
-        np.testing.assert_allclose(N.to("CO2").magnitude, 44 * 298 / 14)
+        np.testing.assert_allclose(CO2.to("N2ON").magnitude, 28 / (44 * 298))
+        np.testing.assert_allclose(N2ON.to("CO2").magnitude, 44 * 298 / 28)
+
+        np.testing.assert_allclose(CO2.to("N2O").magnitude, 1 / 298)
+        np.testing.assert_allclose(N2O.to("CO2").magnitude, 298)
 
 
 def test_context_with_magnitude():
     CO2 = 1 * unit_registry("CO2")
-    N = 1 * unit_registry("N")
+    N2ON = 1 * unit_registry("N2ON")
     with unit_registry.context("AR4GWP100"):
-        np.testing.assert_allclose(CO2.to("N").magnitude, 14 / (44 * 298))
-        np.testing.assert_allclose(N.to("CO2").magnitude, 44 * 298 / 14)
+        np.testing.assert_allclose(CO2.to("N2ON").magnitude, 28 / (44 * 298))
+        np.testing.assert_allclose(N2ON.to("CO2").magnitude, 44 * 298 / 28)
 
 
 def test_context_compound_unit():
     CO2 = 1 * unit_registry("kg CO2 / yr")
-    N = 1 * unit_registry("kg N / yr")
+    N2ON = 1 * unit_registry("kg N2ON / yr")
     with unit_registry.context("AR4GWP100"):
-        np.testing.assert_allclose(CO2.to("kg N / yr").magnitude, 14 / (44 * 298))
-        np.testing.assert_allclose(N.to("kg CO2 / yr").magnitude, 44 * 298 / 14)
+        np.testing.assert_allclose(CO2.to("kg N2ON / yr").magnitude, 28 / (44 * 298))
+        np.testing.assert_allclose(N2ON.to("kg CO2 / yr").magnitude, 44 * 298 / 28)
 
 
 def test_context_dimensionality_error():
     CO2 = unit_registry("CO2")
     with pytest.raises(DimensionalityError):
-        CO2.to("N")
+        CO2.to("N2O")
 
 
 @pytest.mark.parametrize(