Backport PR #38504: REG: DataFrame.shift with axis=1 and CategoricalIndex columns (#38555)

Co-authored-by: jbrockmendel <jbrockmendel@gmail.com>

Author: simonjayhawkins

pandas/core/frame.py

@@ -4572,20 +4572,23 @@ def shift(
         if axis == 1 and periods != 0 and fill_value is lib.no_default and ncols > 0:
             # We will infer fill_value to match the closest column
 
+            # Use a column that we know is valid for our column's dtype GH#38434
+            label = self.columns[0]
+
             if periods > 0:
                 result = self.iloc[:, :-periods]
                 for col in range(min(ncols, abs(periods))):
                     # TODO(EA2D): doing this in a loop unnecessary with 2D EAs
                     # Define filler inside loop so we get a copy
                     filler = self.iloc[:, 0].shift(len(self))
-                    result.insert(0, col, filler, allow_duplicates=True)
+                    result.insert(0, label, filler, allow_duplicates=True)
             else:
                 result = self.iloc[:, -periods:]
                 for col in range(min(ncols, abs(periods))):
                     # Define filler inside loop so we get a copy
                     filler = self.iloc[:, -1].shift(len(self))
                     result.insert(
-                        len(result.columns), col, filler, allow_duplicates=True
+                        len(result.columns), label, filler, allow_duplicates=True
                     )
 
             result.columns = self.columns.copy()

pandas/tests/frame/methods/test_shift.py

@@ -2,7 +2,7 @@
 import pytest
 
 import pandas as pd
-from pandas import DataFrame, Index, Series, date_range, offsets
+from pandas import CategoricalIndex, DataFrame, Index, Series, date_range, offsets
 import pandas._testing as tm
 
 
@@ -292,3 +292,25 @@ def test_shift_dt64values_int_fill_deprecated(self):
 
         expected = DataFrame({"A": [pd.Timestamp(0), pd.Timestamp(0)], "B": df2["A"]})
         tm.assert_frame_equal(result, expected)
+
+    def test_shift_axis1_categorical_columns(self):
+        # GH#38434
+        ci = CategoricalIndex(["a", "b", "c"])
+        df = DataFrame(
+            {"a": [1, 3], "b": [2, 4], "c": [5, 6]}, index=ci[:-1], columns=ci
+        )
+        result = df.shift(axis=1)
+
+        expected = DataFrame(
+            {"a": [np.nan, np.nan], "b": [1, 3], "c": [2, 4]}, index=ci[:-1], columns=ci
+        )
+        tm.assert_frame_equal(result, expected)
+
+        # periods != 1
+        result = df.shift(2, axis=1)
+        expected = DataFrame(
+            {"a": [np.nan, np.nan], "b": [np.nan, np.nan], "c": [1, 3]},
+            index=ci[:-1],
+            columns=ci,
+        )
+        tm.assert_frame_equal(result, expected)