From 10872e30e0ef53712b06b8951f9e224d58b909ae Mon Sep 17 00:00:00 2001
From: Shenyang Cai <sycai@google.com>
Date: Fri, 13 Sep 2024 20:04:29 +0000
Subject: [PATCH] fix: Fix a bug that raises exception when re-indexing columns
 with their original order

---
 bigframes/dataframe.py               |  5 +++++
 tests/system/small/test_dataframe.py | 15 +++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/bigframes/dataframe.py b/bigframes/dataframe.py
index 2ae6aefe1b..c066e5d33a 100644
--- a/bigframes/dataframe.py
+++ b/bigframes/dataframe.py
@@ -1913,6 +1913,11 @@ def _reindex_rows(
     def _reindex_columns(self, columns):
         block = self._block
         new_column_index, indexer = self.columns.reindex(columns)
+
+        if indexer is None:
+            # The new index is the same as the old one. Do nothing.
+            return self
+
         result_cols = []
         for label, index in zip(columns, indexer):
             if index >= 0:
diff --git a/tests/system/small/test_dataframe.py b/tests/system/small/test_dataframe.py
index f51b597650..af56b61448 100644
--- a/tests/system/small/test_dataframe.py
+++ b/tests/system/small/test_dataframe.py
@@ -3664,6 +3664,21 @@ def test_df_reindex_columns(scalars_df_index, scalars_pandas_df_index):
     )
 
 
+def test_df_reindex_columns_with_same_order(scalars_df_index, scalars_pandas_df_index):
+    # First, make sure the two dataframes have the same columns in order.
+    columns = ["int64_col", "int64_too"]
+    bf = scalars_df_index[columns]
+    pd_df = scalars_pandas_df_index[columns]
+
+    bf_result = bf.reindex(columns=columns).to_pandas()
+    pd_result = pd_df.reindex(columns=columns)
+
+    pd.testing.assert_frame_equal(
+        bf_result,
+        pd_result,
+    )
+
+
 def test_df_equals_identical(scalars_df_index, scalars_pandas_df_index):
     unsupported = [
         "geography_col",