BUG: Try to sort result of Index.union rather than guessing sortability

closes #17376

doc/source/whatsnew/v0.21.0.txt

@@ -359,6 +359,7 @@ Indexing
 - Bug in ``.iloc`` when used with inplace addition or assignment and an int indexer on a ``MultiIndex`` causing the wrong indexes to be read from and written to (:issue:`17148`)
 - Bug in ``.isin()`` in which checking membership in empty ``Series`` objects raised an error (:issue:`16991`)
 - Bug in ``CategoricalIndex`` reindexing in which specified indices containing duplicates were not being respected (:issue:`17323`)
+- Bug in the order of the result of ``Index.union()`` when indexes contain tuples (:issue:`17376`)
 
 I/O
 ^^^

pandas/core/indexes/base.py

@@ -2195,35 +2195,21 @@ def union(self, other):
                 value_set = set(self._values)
                 result.extend([x for x in other._values if x not in value_set])
         else:
-            indexer = self.get_indexer(other)
-            indexer, = (indexer == -1).nonzero()
-
+            indexer = np.where(self.get_indexer(other) == -1)[0]
             if len(indexer) > 0:
                 other_diff = algos.take_nd(other._values, indexer,
                                            allow_fill=False)
                 result = _concat._concat_compat((self._values, other_diff))
 
-                try:
-                    self._values[0] < other_diff[0]
-                except TypeError as e:
-                    warnings.warn("%s, sort order is undefined for "
-                                  "incomparable objects" % e, RuntimeWarning,
-                                  stacklevel=3)
-                else:
-                    types = frozenset((self.inferred_type,
-                                       other.inferred_type))
-                    if not types & _unsortable_types:
-                        result.sort()
-
             else:
                 result = self._values
 
-                try:
-                    result = np.sort(result)
-                except TypeError as e:
-                    warnings.warn("%s, sort order is undefined for "
-                                  "incomparable objects" % e, RuntimeWarning,
-                                  stacklevel=3)
+            try:
+                result = sorting.safe_sort(result)
+            except TypeError as e:
+                warnings.warn("%s, sort order is undefined for "
+                              "incomparable objects" % e, RuntimeWarning,
+                              stacklevel=3)
 
         # for subclasses
         return self._wrap_union_result(other, result)

pandas/tests/indexes/test_base.py

@@ -750,8 +750,7 @@ def test_union(self):
         expected = Index(list('ab'), name='A')
         tm.assert_index_equal(union, expected)
 
-        with tm.assert_produces_warning(RuntimeWarning):
-            firstCat = self.strIndex.union(self.dateIndex)
+        firstCat = self.strIndex.union(self.dateIndex)
         secondCat = self.strIndex.union(self.strIndex)
 
         if self.dateIndex.dtype == np.object_:
@@ -1309,29 +1308,26 @@ def test_drop(self):
         expected = Index([1, 2])
         tm.assert_index_equal(dropped, expected)
 
-    def test_tuple_union_bug(self):
-        import pandas
-        import numpy as np
-
+    def test_tuples_intersection_union(self):
         aidx1 = np.array([(1, 'A'), (2, 'A'), (1, 'B'), (2, 'B')],
                          dtype=[('num', int), ('let', 'a1')])
         aidx2 = np.array([(1, 'A'), (2, 'A'), (1, 'B'),
                           (2, 'B'), (1, 'C'), (2, 'C')],
                          dtype=[('num', int), ('let', 'a1')])
 
-        idx1 = pandas.Index(aidx1)
-        idx2 = pandas.Index(aidx2)
+        idx1 = Index(aidx1)
+        idx2 = Index(aidx2)
 
-        # intersection broken?
+        # intersection
         int_idx = idx1.intersection(idx2)
+        expected = idx1  # pandas.Index(sorted(set(idx1) & set(idx2)))
         # needs to be 1d like idx1 and idx2
-        expected = idx1[:4]  # pandas.Index(sorted(set(idx1) & set(idx2)))
         assert int_idx.ndim == 1
         tm.assert_index_equal(int_idx, expected)
 
-        # union broken
+        # GH 17376 (union)
         union_idx = idx1.union(idx2)
-        expected = idx2
+        expected = idx2.sort_values()
         assert union_idx.ndim == 1
         tm.assert_index_equal(union_idx, expected)
 
@@ -1507,13 +1503,19 @@ def test_outer_join_sort(self):
         left_idx = Index(np.random.permutation(15))
         right_idx = tm.makeDateIndex(10)
 
-        with tm.assert_produces_warning(RuntimeWarning):
+        if PY3:
+            with tm.assert_produces_warning(RuntimeWarning):
+                joined = left_idx.join(right_idx, how='outer')
+        else:
             joined = left_idx.join(right_idx, how='outer')
 
         # right_idx in this case because DatetimeIndex has join precedence over
         # Int64Index
-        with tm.assert_produces_warning(RuntimeWarning):
-            expected = right_idx.astype(object).union(left_idx.astype(object))
+        if PY3:
+            with tm.assert_produces_warning(RuntimeWarning):
+                expected = right_idx.astype(object).union(left_idx)
+        else:
+            expected = right_idx.astype(object).union(left_idx)
         tm.assert_index_equal(joined, expected)
 
     def test_nan_first_take_datetime(self):
@@ -1897,10 +1899,7 @@ def test_copy_name(self):
         s1 = Series(2, index=first)
         s2 = Series(3, index=second[:-1])
 
-        warning_type = RuntimeWarning if PY3 else None
-        with tm.assert_produces_warning(warning_type):
-            # Python 3: Unorderable types
-            s3 = s1 * s2
+        s3 = s1 * s2
 
         assert s3.index.name == 'mario'
 
@@ -1933,27 +1932,14 @@ def test_union_base(self):
         first = idx[3:]
         second = idx[:5]
 
-        if PY3:
-            with tm.assert_produces_warning(RuntimeWarning):
-                # unorderable types
-                result = first.union(second)
-                expected = Index(['b', 2, 'c', 0, 'a', 1])
-                tm.assert_index_equal(result, expected)
-        else:
-            result = first.union(second)
-            expected = Index(['b', 2, 'c', 0, 'a', 1])
-            tm.assert_index_equal(result, expected)
+        expected = Index([0, 1, 2, 'a', 'b', 'c'])
+        result = first.union(second)
+        tm.assert_index_equal(result, expected)
 
         # GH 10149
         cases = [klass(second.values)
                  for klass in [np.array, Series, list]]
         for case in cases:
-            if PY3:
-                with tm.assert_produces_warning(RuntimeWarning):
-                    # unorderable types
-                    result = first.union(case)
-                    assert tm.equalContents(result, idx)
-            else:
                 result = first.union(case)
                 assert tm.equalContents(result, idx)
 

pandas/tests/series/test_operators.py

@@ -1198,11 +1198,7 @@ def test_comparison_label_based(self):
             assert_series_equal(result, a[a])
 
         for e in [Series(['z'])]:
-            if compat.PY3:
-                with tm.assert_produces_warning(RuntimeWarning):
-                    result = a[a | e]
-            else:
-                result = a[a | e]
+            result = a[a | e]
             assert_series_equal(result, a[a])
 
         # vs scalars
@@ -1394,24 +1390,12 @@ def test_operators_bitwise(self):
         pytest.raises(TypeError, lambda: s_0123 & [0.1, 4, 3.14, 2])
 
         # s_0123 will be all false now because of reindexing like s_tft
-        if compat.PY3:
-            # unable to sort incompatible object via .union.
-            exp = Series([False] * 7, index=['b', 'c', 'a', 0, 1, 2, 3])
-            with tm.assert_produces_warning(RuntimeWarning):
-                assert_series_equal(s_tft & s_0123, exp)
-        else:
-            exp = Series([False] * 7, index=[0, 1, 2, 3, 'a', 'b', 'c'])
-            assert_series_equal(s_tft & s_0123, exp)
+        exp = Series([False] * 7, index=[0, 1, 2, 3, 'a', 'b', 'c'])
+        assert_series_equal(s_tft & s_0123, exp)
 
         # s_tft will be all false now because of reindexing like s_0123
-        if compat.PY3:
-            # unable to sort incompatible object via .union.
-            exp = Series([False] * 7, index=[0, 1, 2, 3, 'b', 'c', 'a'])
-            with tm.assert_produces_warning(RuntimeWarning):
-                assert_series_equal(s_0123 & s_tft, exp)
-        else:
-            exp = Series([False] * 7, index=[0, 1, 2, 3, 'a', 'b', 'c'])
-            assert_series_equal(s_0123 & s_tft, exp)
+        exp = Series([False] * 7, index=[0, 1, 2, 3, 'a', 'b', 'c'])
+        assert_series_equal(s_0123 & s_tft, exp)
 
         assert_series_equal(s_0123 & False, Series([False] * 4))
         assert_series_equal(s_0123 ^ False, Series([False, True, True, True]))