DEPR: Deprecate outer ufunc in Series.__array_ufunc__

Closes pandas-dev#27186

Author: TomAugspurger

doc/source/getting_started/dsintro.rst

@@ -753,6 +753,11 @@ The ufunc is applied to the underlying array in a Series.
    ser = pd.Series([1, 2, 3, 4])
    np.exp(ser)
 
+.. versionchanged:: 0.25.0
+
+   When multiple ``Series`` are passed to a ufunc, they are aligned before
+   performing the operation.
+
 Like other parts of the library, pandas will automatically align labeled inputs
 as part of a ufunc with multiple inputs. For example, using :meth:`numpy.remainder`
 on two :class:`Series` with differently ordered labels will align before the operation.

doc/source/whatsnew/v0.25.0.rst

@@ -755,6 +755,7 @@ Other deprecations
 - The :meth:`Series.get_values`, :meth:`DataFrame.get_values`, :meth:`Index.get_values`,
   :meth:`SparseArray.get_values` and :meth:`Categorical.get_values` methods are deprecated.
   One of ``np.asarray(..)`` or :meth:`~Series.to_numpy` can be used instead (:issue:`19617`).
+- The 'outer' method on NumPy ufuncs, e.g. ``np.subtract.outer`` has been deprecated on :class:`Series` objects. Convert the input to an array with :attr:`Series.array` first (:issue:`27186`)
 - :meth:`Timedelta.resolution` is deprecated and replaced with :meth:`Timedelta.resolution_string`.  In a future version, :meth:`Timedelta.resolution` will be changed to behave like the standard library :attr:`timedelta.resolution` (:issue:`21344`)
 - :func:`read_table` has been undeprecated. (:issue:`25220`)
 - :attr:`Index.dtype_str` is deprecated. (:issue:`18262`)

pandas/core/series.py

@@ -780,6 +780,19 @@ def __array_ufunc__(
         def construct_return(result):
             if lib.is_scalar(result):
                 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)
+                return result
             return self._constructor(result,
                                      index=index,
                                      name=name,

pandas/tests/series/test_ufunc.py

@@ -310,3 +310,18 @@ def __repr__(self):
     result = np.add(s, Thing(1))
     expected = pd.Series([Thing(2), Thing(3)])
     tm.assert_series_equal(result, expected)
+
+
+def test_outer():
+    # https://github.com/pandas-dev/pandas/issues/27186
+    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]
+    ])
+    tm.assert_numpy_array_equal(result, expected)