BUG: MultiIndex union setting wrong sortorder parameter (#44762)

Author: phofl

doc/source/whatsnew/v1.4.0.rst

@@ -714,6 +714,7 @@ MultiIndex
 - Bug in :meth:`MultiIndex.get_loc` where the first level is a :class:`DatetimeIndex` and a string key is passed (:issue:`42465`)
 - Bug in :meth:`MultiIndex.reindex` when passing a ``level`` that corresponds to an ``ExtensionDtype`` level (:issue:`42043`)
 - Bug in :meth:`MultiIndex.get_loc` raising ``TypeError`` instead of ``KeyError`` on nested tuple (:issue:`42440`)
+- Bug in :meth:`MultiIndex.union` setting wrong ``sortorder`` causing errors in subsequent indexing operations with slices (:issue:`44752`)
 - Bug in :meth:`MultiIndex.putmask` where the other value was also a :class:`MultiIndex` (:issue:`43212`)
 -
 

pandas/core/indexes/multi.py

@@ -3593,7 +3593,7 @@ def _union(self, other, sort) -> MultiIndex:
             rvals = other._values.astype(object, copy=False)
             result = lib.fast_unique_multiple([self._values, rvals], sort=sort)
 
-        return MultiIndex.from_arrays(zip(*result), sortorder=0, names=result_names)
+        return MultiIndex.from_arrays(zip(*result), sortorder=None, names=result_names)
 
     def _is_comparable_dtype(self, dtype: DtypeObj) -> bool:
         return is_object_dtype(dtype)

pandas/tests/indexing/multiindex/test_loc.py

@@ -379,6 +379,26 @@ def test_multiindex_setitem_columns_enlarging(self, indexer, exp_value):
         )
         tm.assert_frame_equal(df, expected)
 
+    def test_sorted_multiindex_after_union(self):
+        # GH#44752
+        midx = MultiIndex.from_product(
+            [pd.date_range("20110101", periods=2), Index(["a", "b"])]
+        )
+        ser1 = Series(1, index=midx)
+        ser2 = Series(1, index=midx[:2])
+        df = pd.concat([ser1, ser2], axis=1)
+        expected = df.copy()
+        result = df.loc["2011-01-01":"2011-01-02"]
+        tm.assert_frame_equal(result, expected)
+
+        df = DataFrame({0: ser1, 1: ser2})
+        result = df.loc["2011-01-01":"2011-01-02"]
+        tm.assert_frame_equal(result, expected)
+
+        df = pd.concat([ser1, ser2.reindex(ser1.index)], axis=1)
+        result = df.loc["2011-01-01":"2011-01-02"]
+        tm.assert_frame_equal(result, expected)
+
 
 @pytest.mark.parametrize(
     "indexer, pos",