BUG: do not fail when stack()ing unsortable level

closes pandas-dev#18310

Author: toobaz

doc/source/whatsnew/v0.22.0.txt

@@ -142,7 +142,8 @@ Sparse
 Reshaping
 ^^^^^^^^^
 
--
+- Bug in :func:`DataFrame.stack` which fails trying to sort mixed type levels under Python 3 (:issue:`18310`)
+
 -
 -
 

pandas/core/indexes/multi.py

@@ -1292,19 +1292,19 @@ def _sort_levels_monotonic(self):
 
         for lev, lab in zip(self.levels, self.labels):
 
-            if lev.is_monotonic:
-                new_levels.append(lev)
-                new_labels.append(lab)
-                continue
-
-            # indexer to reorder the levels
-            indexer = lev.argsort()
-            lev = lev.take(indexer)
+            if not lev.is_monotonic:
+                try:
+                    # indexer to reorder the levels
+                    indexer = lev.argsort()
+                except TypeError:
+                    pass
+                else:
+                    lev = lev.take(indexer)
 
-            # indexer to reorder the labels
-            indexer = _ensure_int64(indexer)
-            ri = lib.get_reverse_indexer(indexer, len(indexer))
-            lab = algos.take_1d(ri, lab)
+                    # indexer to reorder the labels
+                    indexer = _ensure_int64(indexer)
+                    ri = lib.get_reverse_indexer(indexer, len(indexer))
+                    lab = algos.take_1d(ri, lab)
 
             new_levels.append(lev)
             new_labels.append(lab)

pandas/tests/frame/test_reshape.py

@@ -133,6 +133,30 @@ def test_stack_unstack(self):
         assert_frame_equal(unstacked_cols.T, df)
         assert_frame_equal(unstacked_cols_df['bar'].T, df)
 
+    def test_stack_mixed_level(self):
+        # GH 18310
+        levels = [range(3), [3, 'a', 'b'], [1, 2]]
+
+        # flat columns:
+        df = DataFrame(1, index=levels[0], columns=levels[1])
+        result = df.stack()
+        expected = Series(1, index=MultiIndex.from_product(levels[:2]))
+        assert_series_equal(result, expected)
+
+        # MultiIndex columns:
+        df = DataFrame(1, index=levels[0],
+                       columns=MultiIndex.from_product(levels[1:]))
+        result = df.stack(1)
+        expected = DataFrame(1, index=MultiIndex.from_product([levels[0],
+                                                               levels[2]]),
+                             columns=levels[1])
+        assert_frame_equal(result, expected)
+
+        # as above, but used labels in level are actually of homogeneous type
+        result = df[['a', 'b']].stack(1)
+        expected = expected[['a', 'b']]
+        assert_frame_equal(result, expected)
+
     def test_unstack_fill(self):
 
         # GH #9746: fill_value keyword argument for Series