Provide abstract .Quantity.to_series()

doc/whatsnew.rst

@@ -9,6 +9,14 @@ What's new
 .. Next release
 .. ============
 
+v1.2.1 (2021-03-08)
+===================
+
+Bug fixes
+---------
+
+- Provide abstract :class:`.Quantity.to_series` method for type checking in packages that depend on :mod:`genno`.
+
 v1.2.0 (2021-03-08)
 ===================
 

genno/core/quantity.py

@@ -16,10 +16,16 @@ def __new__(cls, *args, **kwargs):
         # Use _get_class() to retrieve either AttrSeries or SparseDataArray
         return object.__new__(Quantity._get_class(cls))
 
+    def to_series(self) -> pd.Series:
+        """Like :meth:`xarray.DataArray.to_series`."""
+        # Provided only for type-checking in other packages. AttrSeries implements;
+        # SparseDataArray uses the xr.DataArray method.
+        raise RuntimeError
+
     @classmethod
     def from_series(cls, series, sparse=True):
         """Convert `series` to the Quantity class given by :data:`.CLASS`."""
-        # NB signature is the same as xr.DataArray.from_series()
+        # NB signature is the same as xr.DataArray.from_series(); except sparse=True
         assert sparse
         return cls._get_class().from_series(series, sparse)