BUG: CategoricalIndex.format

Author: topper-123

doc/source/whatsnew/v1.1.0.rst

@@ -1095,6 +1095,7 @@ I/O
 - Bug in :func:`read_excel` where datetime values are used in the header in a :class:`MultiIndex` (:issue:`34748`)
 - :func:`read_excel` no longer takes ``**kwds`` arguments. This means that passing in the keyword argument ``chunksize`` now raises a ``TypeError`` (previously raised a ``NotImplementedError``), while passing in the keyword argument ``encoding`` now raises a ``TypeError`` (:issue:`34464`)
 - Bug in :meth:`DataFrame.to_records` was incorrectly losing timezone information in timezone-aware ``datetime64`` columns (:issue:`32535`)
+- Bug in :meth:`DataFrame.to_string` when :attr:`DataFrame.columns` is a :class:`CategoricalIndex`  (:issue:`35439`)
 
 Plotting
 ^^^^^^^^

pandas/core/indexes/category.py

@@ -20,7 +20,7 @@
     pandas_dtype,
 )
 from pandas.core.dtypes.dtypes import CategoricalDtype
-from pandas.core.dtypes.missing import is_valid_nat_for_dtype, isna
+from pandas.core.dtypes.missing import is_valid_nat_for_dtype, isna, notna
 
 from pandas.core import accessor
 from pandas.core.algorithms import take_1d
@@ -348,12 +348,12 @@ def _format_attrs(self):
         return attrs
 
     def _format_with_header(self, header, na_rep="NaN") -> List[str]:
-        from pandas.io.formats.format import format_array
+        from pandas.io.formats.printing import pprint_thing
 
-        formatted_values = format_array(
-            self._values, formatter=None, na_rep=na_rep, justify="left"
-        )
-        result = ibase.trim_front(formatted_values)
+        result = [
+            pprint_thing(x, escape_chars=("\t", "\r", "\n")) if notna(x) else na_rep
+            for x in self._values
+        ]
         return header + result
 
     # --------------------------------------------------------------------

pandas/tests/io/formats/test_format.py

@@ -2141,6 +2141,15 @@ def test_dict_entries(self):
         assert "'a': 1" in val
         assert "'b': 2" in val
 
+    def test_categorical_columns(self):
+        # GH35439
+        data = [[4, 2], [3, 2], [4, 3]]
+        cols = ["aaaaaaaaa", "b"]
+        df = pd.DataFrame(data, columns=cols)
+        df_cat_cols = pd.DataFrame(data, columns=pd.CategoricalIndex(cols))
+
+        assert df.to_string() == df_cat_cols.to_string()
+
     def test_period(self):
         # GH 12615
         df = pd.DataFrame(