API: Disallow DataFrame other for Series flex ops

Author: jbrockmendel

doc/source/whatsnew/v3.0.0.rst

@@ -655,6 +655,7 @@ Other API changes
   an empty ``RangeIndex`` or empty ``Index`` with object dtype when determining
   the dtype of the resulting Index (:issue:`60797`)
 - :class:`IncompatibleFrequency` now subclasses ``TypeError`` instead of ``ValueError``. As a result, joins with mismatched frequencies now cast to object like other non-comparable joins, and arithmetic with indexes with mismatched frequencies align (:issue:`55782`)
+- :class:`Series` "flex" methods like :meth:`Series.add` no longer allow passing a :class:`DataFrame` for ``other``; use the DataFrame reversed method instead (:issue:`46179`)
 - :meth:`CategoricalIndex.append` no longer attempts to cast different-dtype indexes to the caller's dtype (:issue:`41626`)
 - :meth:`ExtensionDtype.construct_array_type` is now a regular method instead of a ``classmethod`` (:issue:`58663`)
 - Comparison operations between :class:`Index` and :class:`Series` now consistently return :class:`Series` regardless of which object is on the left or right (:issue:`36759`)

pandas/core/series.py

@@ -6069,6 +6069,12 @@ def _flex_method(self, other, op, *, level=None, fill_value=None, axis: Axis = 0
             result = self._binop(other, op, level=level, fill_value=fill_value)
             result._name = res_name
             return result
+        elif isinstance(other, ABCDataFrame):
+            # GH#46179
+            raise TypeError(
+                f"Series.{op.__name__.strip('_')} does not support a DataFrame "
+                f"`other`. Use df.{op.__name__.strip('_')}(ser) instead."
+            )
         else:
             if fill_value is not None:
                 if isna(other):

pandas/tests/series/test_arithmetic.py

@@ -156,6 +156,24 @@ def _check_fill(meth, op, a, b, fill_value=0):
         # should accept axis=0 or axis='rows'
         op(a, b, axis=0)
 
+    def test_flex_disallows_dataframe(self):
+        # GH#46179
+        df = pd.DataFrame(
+            {2010: [1], 2020: [3]},
+            index=pd.MultiIndex.from_product([["a"], ["b"]], names=["scen", "mod"]),
+        )
+
+        ser = Series(
+            [10.0, 20.0, 30.0],
+            index=pd.MultiIndex.from_product(
+                [["a"], ["b"], [0, 1, 2]], names=["scen", "mod", "id"]
+            ),
+        )
+
+        msg = "Series.add does not support a DataFrame `other`"
+        with pytest.raises(TypeError, match=msg):
+            ser.add(df, axis=0)
+
 
 class TestSeriesArithmetic:
     # Some of these may end up in tests/arithmetic, but are not yet sorted