BUG: Fix MultiIndex DataFrame to_csv() segfault

doc/source/whatsnew/v0.25.0.rst

@@ -254,6 +254,7 @@ Performance Improvements
 - Improved performance of :meth:`Series.map` for dictionary mappers on categorical series by mapping the categories instead of mapping all values (:issue:`23785`)
 
 .. _whatsnew_0250.bug_fixes:
+- Fixed MultiIndex DataFrame to_csv segfault (:issue:`26303`)
 
 Bug Fixes
 ~~~~~~~~~

pandas/core/indexes/multi.py

@@ -946,7 +946,7 @@ def _format_native_types(self, na_rep='nan', **kwargs):
             new_codes.append(level_codes)
 
         if len(new_levels) == 1:
-            return Index(new_levels[0])._format_native_types()
+            return Index(new_levels[0][new_codes[0]])._format_native_types()
         else:
             # reconstruct the multi-index
             mi = MultiIndex(levels=new_levels, codes=new_codes,

pandas/tests/frame/test_to_csv.py

@@ -874,6 +874,18 @@ def test_to_csv_index_no_leading_comma(self):
         expected = tm.convert_rows_list_to_csv_str(expected_rows)
         assert buf.getvalue() == expected
 
+    def test_to_csv_multi_index(self):
+        # see gh-26303
+        index = pd.Index([(1,), (2,), (3,)])
+        df = pd.DataFrame([[1, 2, 3]], columns=index)
+        with ensure_clean() as path:
+            df = df.reindex(columns=[(1,), (3,)])
+            df.to_csv(path, line_terminator='\n')
+            expected = b",1,3\n0,1,3\n"
+
+            with open(path, mode='rb') as f:
+                assert f.read() == expected
+
     def test_to_csv_line_terminators(self):
         # see gh-20353
         df = DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6]},