[WIP]: API: Change default for Index.union sort

Closes pandas-dev#24959

pandas/core/indexes/base.py

@@ -2245,18 +2245,34 @@ def _get_reconciled_name_object(self, other):
             return self._shallow_copy(name=name)
         return self
 
-    def union(self, other, sort=True):
+    def union(self, other, sort=None):
         """
         Form the union of two Index objects.
 
         Parameters
         ----------
         other : Index or array-like
-        sort : bool, default True
-            Sort the resulting index if possible
+        sort : bool or None, default None
+            Whether to sort the resulting Index.
+
+            * None : Sort the result, except when
+
+              1. `self` and `other` are equal.
+              2. `self` or `other` has length 0.
+              3. Some values in `self` or `other` cannot be compared.
+                 A RuntimeWarning is issued in this case.
+
+            * True : sort the result. A TypeError is raised when the
+              values cannot be compared.
+            * False : do not sort the result.
 
             .. versionadded:: 0.24.0
 
+            .. versionchanged:: 0.24.0
+
+               Changed the default `sort` to None, matching the
+               behavior of pandas 0.23.4 and earlier.
+
         Returns
         -------
         union : Index
@@ -2273,10 +2289,16 @@ def union(self, other, sort=True):
         other = ensure_index(other)
 
         if len(other) == 0 or self.equals(other):
-            return self._get_reconciled_name_object(other)
+            result = self._get_reconciled_name_object(other)
+            if sort:
+                result = result.sort_values()
+            return result
 
         if len(self) == 0:
-            return other._get_reconciled_name_object(self)
+            result = other._get_reconciled_name_object(self)
+            if sort:
+                result = result.sort_values()
+            return result
 
         # TODO: is_dtype_union_equal is a hack around
         # 1. buggy set ops with duplicates (GH #13432)
@@ -2319,13 +2341,16 @@ def union(self, other, sort=True):
             else:
                 result = lvals
 
-            if sort:
+            if sort is None:
                 try:
                     result = sorting.safe_sort(result)
                 except TypeError as e:
                     warnings.warn("{}, sort order is undefined for "
                                   "incomparable objects".format(e),
                                   RuntimeWarning, stacklevel=3)
+            elif sort:
+                # raise if not sortable.
+                result = sorting.safe_sort(result)
 
         # for subclasses
         return self._wrap_setop_result(other, result)

pandas/tests/indexes/test_base.py

@@ -794,6 +794,52 @@ def test_union(self, sort):
             tm.assert_index_equal(union, everything.sort_values())
         assert tm.equalContents(union, everything)
 
+    def test_union_sort_other_equal(self):
+        a = pd.Index([1, 0, 2])
+        # default, sort=None
+        result = a.union(a)
+        tm.assert_index_equal(result, a)
+
+        # sort=True
+        result = a.union(a, sort=True)
+        expected = pd.Index([0, 1, 2])
+        tm.assert_index_equal(result, expected)
+
+        # sort=False
+        result = a.union(a, sort=False)
+        tm.assert_index_equal(result, a)
+
+    def test_union_sort_other_empty(self):
+        a = pd.Index([1, 0, 2])
+        # default, sort=None
+        tm.assert_index_equal(a.union(a[:0]), a)
+        tm.assert_index_equal(a[:0].union(a), a)
+
+        # sort=True
+        expected = pd.Index([0, 1, 2])
+        tm.assert_index_equal(a.union(a[:0], sort=True), expected)
+        tm.assert_index_equal(a[:0].union(a, sort=True), expected)
+
+        # sort=False
+        tm.assert_index_equal(a.union(a[:0], sort=False), a)
+        tm.assert_index_equal(a[:0].union(a, sort=False), a)
+
+    def test_union_sort_other_incomparable(self):
+        a = pd.Index([1, pd.Timestamp('2000')])
+        # default, sort=None
+        with tm.assert_produces_warning(RuntimeWarning):
+            result = a.union(a[:1])
+
+        tm.assert_index_equal(result, a)
+
+        # sort=True
+        with pytest.raises(TypeError, match='.*'):
+            a.union(a[:1], sort=True)
+
+        # sort=False
+        result = a.union(a[:1], sort=False)
+        tm.assert_index_equal(result, a)
+
     @pytest.mark.parametrize("klass", [
         np.array, Series, list])
     @pytest.mark.parametrize("sort", [True, False])