API: Index.__cmp__(Series) return NotImplemented

doc/source/whatsnew/v3.0.0.rst

@@ -414,6 +414,7 @@ Other API changes
 - Index set operations (like union or intersection) will now ignore the dtype of
   an empty ``RangeIndex`` or empty ``Index`` with object dtype when determining
   the dtype of the resulting Index (:issue:`60797`)
+- 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`)
 - Numpy functions like ``np.isinf`` that return a bool dtype when called on a :class:`Index` object now return a bool-dtype :class:`Index` instead of ``np.ndarray`` (:issue:`52676`)
 
 .. ---------------------------------------------------------------------------

pandas/core/ops/common.py

@@ -56,20 +56,14 @@ def _unpack_zerodim_and_defer(method: F, name: str) -> F:
     -------
     method
     """
-    stripped_name = name.removeprefix("__").removesuffix("__")
-    is_cmp = stripped_name in {"eq", "ne", "lt", "le", "gt", "ge"}
 
     @wraps(method)
     def new_method(self, other):
-        if is_cmp and isinstance(self, ABCIndex) and isinstance(other, ABCSeries):
-            # For comparison ops, Index does *not* defer to Series
-            pass
-        else:
-            prio = getattr(other, "__pandas_priority__", None)
-            if prio is not None:
-                if prio > self.__pandas_priority__:
-                    # e.g. other is DataFrame while self is Index/Series/EA
-                    return NotImplemented
+        prio = getattr(other, "__pandas_priority__", None)
+        if prio is not None:
+            if prio > self.__pandas_priority__:
+                # e.g. other is DataFrame while self is Index/Series/EA
+                return NotImplemented
 
         other = item_from_zerodim(other)
 

pandas/tests/arithmetic/common.py

@@ -111,24 +111,19 @@ def xbox2(x):
             return x.astype(bool)
         return x
 
-    # rev_box: box to use for reversed comparisons
-    rev_box = xbox
-    if isinstance(right, Index) and isinstance(left, Series):
-        rev_box = np.array
-
     result = xbox2(left == right)
     expected = xbox(np.zeros(result.shape, dtype=np.bool_))
 
     tm.assert_equal(result, expected)
 
     result = xbox2(right == left)
-    tm.assert_equal(result, rev_box(expected))
+    tm.assert_equal(result, xbox(expected))
 
     result = xbox2(left != right)
     tm.assert_equal(result, ~expected)
 
     result = xbox2(right != left)
-    tm.assert_equal(result, rev_box(~expected))
+    tm.assert_equal(result, xbox(~expected))
 
     msg = "|".join(
         [

pandas/tests/arithmetic/test_datetime64.py

@@ -770,11 +770,18 @@ def test_dti_cmp_tdi_tzawareness(self, other):
 
         result = dti == other
         expected = np.array([False] * 10)
-        tm.assert_numpy_array_equal(result, expected)
+        if isinstance(other, Series):
+            tm.assert_series_equal(result, Series(expected, index=other.index))
+        else:
+            tm.assert_numpy_array_equal(result, expected)
 
         result = dti != other
         expected = np.array([True] * 10)
-        tm.assert_numpy_array_equal(result, expected)
+        if isinstance(other, Series):
+            tm.assert_series_equal(result, Series(expected, index=other.index))
+        else:
+            tm.assert_numpy_array_equal(result, expected)
+
         msg = "Invalid comparison between"
         with pytest.raises(TypeError, match=msg):
             dti < other

pandas/tests/indexes/multi/test_equivalence.py

@@ -64,8 +64,8 @@ def test_equals_op(idx):
     with pytest.raises(ValueError, match="Lengths must match"):
         index_a == series_b
 
-    tm.assert_numpy_array_equal(index_a == series_a, expected1)
-    tm.assert_numpy_array_equal(index_a == series_c, expected2)
+    tm.assert_series_equal(index_a == series_a, Series(expected1))
+    tm.assert_series_equal(index_a == series_c, Series(expected2))
 
     # cases where length is 1 for one of them
     with pytest.raises(ValueError, match="Lengths must match"):

pandas/tests/indexes/test_old_base.py

@@ -560,8 +560,8 @@ def test_equals_op(self, simple_index):
         with pytest.raises(ValueError, match=msg):
             index_a == series_b
 
-        tm.assert_numpy_array_equal(index_a == series_a, expected1)
-        tm.assert_numpy_array_equal(index_a == series_c, expected2)
+        tm.assert_series_equal(index_a == series_a, Series(expected1))
+        tm.assert_series_equal(index_a == series_c, Series(expected2))
 
         # cases where length is 1 for one of them
         with pytest.raises(ValueError, match="Lengths must match"):