BUG: Handle 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.
@@ -788,6 +793,13 @@ NumPy ufuncs are safe to apply to :class:`Series` backed by non-ndarray arrays,
 for example :class:`SparseArray` (see :ref:`sparse.calculation`). If possible,
 the ufunc is applied without converting the underlying data to an ndarray.
 
+If the ``outer`` method is used, a NumPy ndarray is returned.
+
+.. ipython:: python
+
+   np.subtract.outer(ser, ser)
+
+
 Console display
 ~~~~~~~~~~~~~~~
 

pandas/core/series.py

@@ -780,6 +780,9 @@ def __array_ufunc__(
         def construct_return(result):
             if lib.is_scalar(result):
                 return result
+            elif result.ndim > 1:
+                # e.g. np.subtract.outer
+                return result
             return self._constructor(result,
                                      index=index,
                                      name=name,

pandas/tests/series/test_ufunc.py

@@ -310,3 +310,17 @@ 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])
+
+    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)