query: Refactor DBColumn.

Avoid an issubclass check on every call, and make it easier for subclasses
to override part of the call implementation.

conservancy_beancount/reports/query.py

@@ -169,12 +169,19 @@ class DBColumn(bc_query_compile.EvalColumn):
     _db_cursor: ClassVar[sqlite3.Cursor]
     _db_query: ClassVar[str]
     _dtype: ClassVar[Type] = set
+    _return: ClassVar[Callable[['DBColumn'], object]]
     __intypes__ = [Posting]
 
     @classmethod
     def with_db(cls, connection: sqlite3.Connection) -> Type['DBColumn']:
         return type(cls.__name__, (cls,), {'_db_cursor': connection.cursor()})
 
+    def __init_subclass__(cls) -> None:
+        if issubclass(cls._dtype, set):
+            cls._return = cls._return_set
+        else:
+            cls._return = cls._return_scalar
+
     def __init__(self, colname: Optional[str]=None) -> None:
         if not hasattr(self, '_db_cursor'):
             if colname is None:
@@ -186,14 +193,17 @@ class DBColumn(bc_query_compile.EvalColumn):
         entity = meta.get('entity')
         return entity if isinstance(entity, str) else '\0'
 
+    def _return_scalar(self) -> object:
+        row = self._db_cursor.fetchone()
+        return self._dtype() if row is None else self._dtype(row[0])
+
+    def _return_set(self) -> object:
+        return self._dtype(value for value, in self._db_cursor)
+
     def __call__(self, context: PostingContext) -> object:
         entity = self._entity(ContextMeta(context))
         self._db_cursor.execute(self._db_query, (entity,))
-        if issubclass(self._dtype, set):
-            return self._dtype(value for value, in self._db_cursor)
-        else:
-            row = self._db_cursor.fetchone()
-            return self._dtype() if row is None else self._dtype(row[0])
+        return self._return()
 
 
 class DBEmail(DBColumn):