Fix pint regression

CHANGELOG.rst

@@ -1,6 +1,8 @@
 Changelog
 =========
 
+- (`#40 <https://github.com/openscm/openscm-units/pull/40>`_) Fixed broken definition of ppm, caused by regression in Pint where `'ppm' was added to Pint <https://github.com/hgrecco/pint/pull/1661>`_
+
 v0.5.1
 ------
 

src/openscm_units/_unit_registry.py

@@ -302,7 +302,7 @@
 }
 
 
-class ScmUnitRegistry(pint.UnitRegistry):
+class ScmUnitRegistry(pint.UnitRegistry):  # pylint: disable=too-many-ancestors
     """
     Unit registry class.
 
@@ -332,6 +332,8 @@ def __init__(self, *args, metric_conversions=None, **kwargs):
             Passed to the ``__init__`` method of the super class
         """
         self._metric_conversions = metric_conversions
+        # If we didn't call init here, we wouldn't need to rebuild the cache
+        # below but that also feels like a bad pattern
         super().__init__(*args, **kwargs)
 
     def add_standards(self):
@@ -354,9 +356,12 @@ def add_standards(self):
             "Tt = 1000000000000 * t"
         )  # since Tt is used for "tex" in the defaults
 
-        self.define("ppt = [concentrations]")
-        self.define("ppb = 1000 * ppt")
-        self.define("ppm = 1000 * ppb")
+        self.define("ppm = [concentrations]")
+        self.define("ppb = ppm / 1000")
+        self.define("ppt = ppb / 1000")
+        # Have to rebuild cache to get right units for ppm as it is defined in
+        # pint
+        self._build_cache()
 
     def enable_contexts(self, *names_or_contexts, **kwargs):
         """