Use StringIO instead of capturing sys out

Author: kinow

pandas/tests/io/formats/test_to_csv.py

@@ -548,10 +548,10 @@ def test_to_csv_na_rep_long_string(self, capsys):
                          'mynull',
                          'mynull',
                          'mynull']
-        expected_ascii = tm.convert_rows_list_to_csv_str(expected_rows)
+        expected = tm.convert_rows_list_to_csv_str(expected_rows)
 
-        df.to_csv(sys.stdout, index=False, na_rep='mynull', encoding='ascii')
-        captured = capsys.readouterr()
+        csv_data = StringIO()
+        df.to_csv(csv_data, index=False, na_rep='mynull', encoding='ascii')
+        result = csv_data.getvalue()
 
-        assert captured.out == expected_ascii
-        assert not sys.stdout.closed
+        assert expected == result