Allow custom units to be registered with Pint

datasette/app.py

@@ -1186,6 +1186,11 @@ class Datasette:
                 }
         return self._inspect
 
+    def register_custom_units(self):
+        "Register any custom units defined in the metadata.json with Pint"
+        for unit in self.metadata.get('custom_units', []):
+            ureg.define(unit)
+
     def app(self):
         app = Sanic(__name__)
         default_templates = str(app_root / 'datasette' / 'templates')
@@ -1230,6 +1235,8 @@ class Datasette:
             '/<db_name:[^/]+>/<table:[^/]+?>/<pk_path:[^/]+?><as_json:(\.jsono?)?$>'
         )
 
+        self.register_custom_units()
+
         @app.exception(Exception)
         def on_exception(request, exception):
             title = None

docs/metadata.rst

@@ -79,7 +79,14 @@ Column units are configured in the metadata like so::
     }
 
 Units are interpreted using Pint_, and you can see the full list of available units in
-Pint's `unit registry`_.
+Pint's `unit registry`_. You can also add custom units to the metadata, which will be
+registered with Pint::
+    
+    {
+        "custom_units": [
+            "decibel = [] = dB"
+        ]
+    }
 
 .. _Pint: https://pint.readthedocs.io/
 .. _unit registry: https://github.com/hgrecco/pint/blob/master/pint/default_en.txt