Alias fix up

changelog/53.fix.md

@@ -0,0 +1,20 @@
+Fixed accidental aliasing of "yr" to "a".
+
+We had accidentally assigned "yr" to be an alias for "a".
+This meant we had the following behaviour
+
+```python
+>>> from openscm_units import unit_registry
+>>> val = unit_registry.Quantity(1, "yr")
+>>> val
+<Quantity(1, 'a')>
+```
+
+This PR fixes this so that if you pass in "yr", it stays as yr i.e. you get
+
+```python
+>>> from openscm_units import unit_registry
+>>> val = unit_registry.Quantity(1, "yr")
+>>> val
+<Quantity(1, 'yr')>
+```

src/openscm_units/_unit_registry.py

@@ -232,7 +232,8 @@ def add_standards(self) -> None:
 
         self._add_gases({x: x for x in MIXTURES})
 
-        self.define("a = 1 * year = annum = yr")
+        self.define("yr = 1 * year")
+        self.define("a = 1 * year = annum")
         self.define("h = hour")
         self.define("d = day")
         self.define("degreeC = degC")

tests/unit/test_units.py

@@ -380,3 +380,31 @@ def test_split_invalid():
         match="Mixture has dimensionality 2 != 1, which is not supported.",
     ):
         unit_registry.split_gas_mixture(1 * unit_registry("CFC400") ** 2)
+
+
+@pytest.mark.parametrize("inp", ("yr",))
+def test_no_autoconversion(inp):
+    """
+    Make sure that the units are not automatically converted to some other value
+
+    The auto-conversion happens if pint thinks that the units are an alias
+    for something else.
+    See {py:func}`test_aliases`.
+    """
+    res = unit_registry.Quantity(1, inp)
+
+    assert str(res.units) == inp
+
+
+@pytest.mark.parametrize(
+    "alias, exp",
+    (
+        ("annum", "a"),
+        ("degC", "degree_Celsius"),
+    ),
+)
+def test_aliases(alias, exp):
+    """Test our aliases behave as expected"""
+    res = unit_registry.Quantity(1, alias)
+
+    assert str(res.units) == exp