Backport PR #31729: BUG: Fix to_excel writers handling of cols (#31764)

Author: meeseeksmachine

doc/source/whatsnew/v1.0.2.rst

@@ -15,7 +15,7 @@ including other versions of pandas.
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
 
--
+- Fixed regression in :meth:`DataFrame.to_excel` when ``columns`` kwarg is passed (:issue:`31677`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/io/formats/excel.py

@@ -400,7 +400,7 @@ def __init__(
                 # Deprecated in GH#17295, enforced in 1.0.0
                 raise KeyError("Not all names specified in 'columns' are found")
 
-            self.df = df
+            self.df = df.reindex(columns=cols)
 
         self.columns = self.df.columns
         self.float_format = float_format

pandas/tests/io/excel/test_writers.py

@@ -1018,6 +1018,27 @@ def test_invalid_columns(self, path):
         ):
             write_frame.to_excel(path, "test1", columns=["C", "D"])
 
+    @pytest.mark.parametrize(
+        "to_excel_index,read_excel_index_col",
+        [
+            (True, 0),  # Include index in write to file
+            (False, None),  # Dont include index in write to file
+        ],
+    )
+    def test_write_subset_columns(self, path, to_excel_index, read_excel_index_col):
+        # GH 31677
+        write_frame = DataFrame({"A": [1, 1, 1], "B": [2, 2, 2], "C": [3, 3, 3]})
+        write_frame.to_excel(
+            path, "col_subset_bug", columns=["A", "B"], index=to_excel_index
+        )
+
+        expected = write_frame[["A", "B"]]
+        read_frame = pd.read_excel(
+            path, "col_subset_bug", index_col=read_excel_index_col
+        )
+
+        tm.assert_frame_equal(expected, read_frame)
+
     def test_comment_arg(self, path):
         # see gh-18735
         #