BUG: fix reducing numpy ufuncs for Series/Index

doc/source/whatsnew/v0.24.0.txt

@@ -1079,6 +1079,7 @@ Numeric
 - Bug in :meth:`DataFrame.astype` to extension dtype may raise ``AttributeError`` (:issue:`22578`)
 - Bug in :class:`DataFrame` with ``timedelta64[ns]`` dtype arithmetic operations with ``ndarray`` with integer dtype incorrectly treating the narray as ``timedelta64[ns]`` dtype (:issue:`23114`)
 - Bug in :meth:`Series.rpow` with object dtype ``NaN`` for ``1 ** NA`` instead of ``1`` (:issue:`22922`).
+- Bug in calling a reducing numpy ufunc on a Series or Index where the scalar result was broadcasted to the original shape of the Series or Index (:issue:`23312`).
 
 Strings
 ^^^^^^^

pandas/core/indexes/base.py

@@ -681,7 +681,7 @@ def __array_wrap__(self, result, context=None):
         """
         Gets called after a ufunc
         """
-        if is_bool_dtype(result):
+        if is_bool_dtype(result) or (getattr(result, 'ndim', None) == 0):
             return result
 
         attrs = self._get_attributes_dict()

pandas/core/series.py

@@ -635,6 +635,8 @@ def __array_wrap__(self, result, context=None):
         """
         Gets called after a ufunc
         """
+        if getattr(result, 'ndim', None) == 0:
+            return result
         return self._constructor(result, index=self.index,
                                  copy=False).__finalize__(self)
 

pandas/tests/arithmetic/test_numeric.py

@@ -790,11 +790,11 @@ def check(series, other):
 
 
 class TestUFuncCompat(object):
-    @pytest.mark.parametrize('holder', [pd.Int64Index, pd.UInt64Index,
-                                        pd.Float64Index, pd.Series])
-    def test_ufunc_coercions(self, holder):
-        idx = holder([1, 2, 3, 4, 5], name='x')
-        box = pd.Series if holder is pd.Series else pd.Index
+    @pytest.mark.parametrize('box', [pd.Int64Index, pd.UInt64Index,
+                                     pd.Float64Index, pd.Series])
+    def test_ufunc_coercions(self, box):
+        idx = box([1, 2, 3, 4, 5], name='x')
+        box = pd.Series if box is pd.Series else pd.Index
 
         result = np.sqrt(idx)
         assert result.dtype == 'f8' and isinstance(result, box)
@@ -833,6 +833,21 @@ def test_ufunc_coercions(self, holder):
         exp = tm.box_expected(exp, box)
         tm.assert_equal(result, exp)
 
+    @pytest.mark.parametrize('box', [pd.Int64Index, pd.UInt64Index,
+                                     pd.Float64Index, pd.Series])
+    def test_ufunc_reduce(self, box):
+        idx = box([1, 2, 3, 4, 5], name='x')
+
+        result = np.add.reduce(idx)
+        # GH23312 result was equal to 15 but boxed as 0-dim array in Index
+        assert not isinstance(result, box)
+        assert result == 15
+
+        result = np.logical_and.reduce(idx)
+        expected = np.array(True)
+        assert not isinstance(result, box)
+        assert tm.assert_numpy_array_equal(result, expected)
+
 
 class TestObjectDtypeEquivalence(object):
     # Tests that arithmetic operations match operations executed elementwise