Handle context=None in convert_unit when input/output GHG species match

pyam/units.py

@@ -39,13 +39,12 @@ def convert_unit(df, current, to, factor=None, registry=None, context=None,
     try:
         # Create a vector pint.Quantity
         qty = registry.Quantity(*qty)
-    except pint.UndefinedUnitError as exc:
-        # *current* might include a GHG species
-        if not context:
-            # Can't do anything without a context
-            raise UndefinedUnitError(*exc.args) from None
-
-        result, to = convert_gwp(context, qty, to)
+    except pint.UndefinedUnitError:
+        try:
+            # *current* might include a GHG species
+            result, to = convert_gwp(context, qty, to)
+        except (AttributeError, ValueError) as exc:
+            raise UndefinedUnitError(to) from None
     except AttributeError:
         # .Quantity() did not exist
         raise TypeError(f'{registry} must be `pint.UnitRegistry`') from None
@@ -65,7 +64,7 @@ def convert_unit(df, current, to, factor=None, registry=None, context=None,
 def convert_gwp(context, qty, to):
     """Helper for :meth:`convert_unit` to perform GWP conversions."""
     # Remove a leading 'gwp_' to produce the metric name
-    metric = context.split('gwp_')[1]
+    metric = context.split('gwp_')[1] if context else context
 
     # Split *to* into a 1- or 3-tuple of str. This allows for *to* to be:
     _to = iam_units.emissions.pattern.split(to, maxsplit=1)

tests/test_units.py

@@ -15,23 +15,17 @@ def get_units_test_df(test_df):
     return df
 
 
-def assert_converted_units(df, current, to, exp, exp_factor=1.0, **kwargs):
+def assert_converted_units(df, current, to, exp, **kwargs):
     # testing for `inplace=False` - converted values and expected unit
     _df = df.convert_unit(current, to, **kwargs, inplace=False)
-    pd.testing.assert_series_equal(
-        _df.data.value,
-        exp * exp_factor,
-        **PRECISE_ARG)
+    pd.testing.assert_series_equal(_df.data.value, exp, **PRECISE_ARG)
     # For GWP conversion with a species name (e.g. 'CO2e'), units are added
     # (e.g. 'Mt CO2e'). Compare using 'in'.
     assert to in _df.data.unit[5]
 
     # testing for `inplace=True` - converted values and expected unit
     df.convert_unit(current, to, **kwargs, inplace=True)
-    pd.testing.assert_series_equal(
-        df.data.value,
-        exp * exp_factor,
-        **PRECISE_ARG)
+    pd.testing.assert_series_equal(df.data.value, exp, **PRECISE_ARG)
     assert to in df.data.unit[5]
 
 
@@ -75,6 +69,12 @@ def test_convert_unit_with_custom_registry(test_df):
     assert_converted_units(df, 'foo', 'baz', exp, registry=ureg)
 
 
+@pytest.mark.parametrize('context, species_current, exp', [
+    ('AR5GWP100', 'CH4', 28),
+    ('AR4GWP100', 'CH4', 25),
+    ('SARGWP100', 'CH4', 21),
+    (None, 'CO2e', 1.),
+])
 @pytest.mark.parametrize('current, to, exp_factor', [
     # exp_factor is used when the conversion includes both a species *and* unit
     # change.
@@ -96,21 +96,23 @@ def test_convert_unit_with_custom_registry(test_df):
     # *current*
     ('t {} / year', 'kt {} / year', 1e-3),
 ])
-def test_convert_unit_with_context(test_df, current, to, exp_factor):
-    # unit conversion with contexts in application registry
+def test_convert_gwp(test_df, context, species_current, current, to, exp,
+                     exp_factor):
+    """Units and GHG species can be converted."""
+    # Prepare input data
     df = test_df.copy()
     df['variable'] = [i.replace('Primary Energy', 'Emissions|CH4')
                       for i in df['variable']]
-    current = current.format('CH4')
+    current = current.format(species_current)
     df['unit'] = current
     to = to.format('CO2e')
 
-    # test conversion for multiple contexts
-    for (c, v) in [('AR5GWP100', 28), ('AR4GWP100', 25), ('SARGWP100', 21)]:
-        exp = test_df.data.value * v
-        assert_converted_units(df.copy(), current, to, exp,
-                               exp_factor=exp_factor,
-                               context=f'gwp_{c}')
+    if context is not None:
+        # pyam-style context
+        context = f'gwp_{context}'
+
+    exp_values = test_df.data.value * exp * exp_factor
+    assert_converted_units(df.copy(), current, to, exp_values, context=context)
 
 
 def test_convert_unit_bad_args(test_pd_df):