DEPR: ufunc.outer (pandas-dev#30104)

Author: jbrockmendel

doc/source/whatsnew/v1.0.0.rst

@@ -604,6 +604,7 @@ or ``matplotlib.Axes.plot``. See :ref:`plotting.formatters` for more.
 - Removed the previously deprecated :meth:`Series.to_dense`, :meth:`DataFrame.to_dense` (:issue:`26684`)
 - Removed the previously deprecated :meth:`Index.dtype_str`, use ``str(index.dtype)`` instead (:issue:`27106`)
 - :meth:`Categorical.ravel` returns a :class:`Categorical` instead of a ``ndarray`` (:issue:`27199`)
+- The 'outer' method on Numpy ufuncs, e.g. ``np.subtract.outer`` operating on :class:`Series` objects is no longer supported, and will raise ``NotImplementedError`` (:issue:`27198`)
 - Removed previously deprecated :meth:`Series.get_dtype_counts` and :meth:`DataFrame.get_dtype_counts` (:issue:`27145`)
 - Changed the default ``fill_value`` in :meth:`Categorical.take` from ``True`` to ``False`` (:issue:`20841`)
 - Changed the default value for the `raw` argument in :func:`Series.rolling().apply() <pandas.core.window.Rolling.apply>`, :func:`DataFrame.rolling().apply() <pandas.core.window.Rolling.apply>`,

pandas/core/series.py

@@ -684,14 +684,8 @@ def construct_return(result):
             elif result.ndim > 1:
                 # e.g. np.subtract.outer
                 if method == "outer":
-                    msg = (
-                        "outer method for ufunc {} is not implemented on "
-                        "pandas objects. Returning an ndarray, but in the "
-                        "future this will raise a 'NotImplementedError'. "
-                        "Consider explicitly converting the Series "
-                        "to an array with '.array' first."
-                    )
-                    warnings.warn(msg.format(ufunc), FutureWarning, stacklevel=3)
+                    # GH#27198
+                    raise NotImplementedError
                 return result
             return self._constructor(result, index=index, name=name, copy=False)
 

pandas/tests/series/test_ufunc.py

@@ -299,7 +299,5 @@ def test_outer():
     s = pd.Series([1, 2, 3])
     o = np.array([1, 2, 3])
 
-    with tm.assert_produces_warning(FutureWarning):
-        result = np.subtract.outer(s, o)
-    expected = np.array([[0, -1, -2], [1, 0, -1], [2, 1, 0]], dtype=np.dtype("int64"))
-    tm.assert_numpy_array_equal(result, expected)
+    with pytest.raises(NotImplementedError):
+        np.subtract.outer(s, o)