From a9fb4c63e348bc25450dffa7ce5c63aba220f30a Mon Sep 17 00:00:00 2001
From: Shuowei Li <shuowei@google.com>
Date: Thu, 26 Jun 2025 04:27:14 +0000
Subject: [PATCH 1/2] Add item assignment for bigframes dataframe

---
 bigframes/series.py               |  4 ++
 tests/system/small/test_series.py | 63 +++++++++++++++++++++++++++++++
 2 files changed, 67 insertions(+)

diff --git a/bigframes/series.py b/bigframes/series.py
index ae6cd7b2ad..ebc2913f78 100644
--- a/bigframes/series.py
+++ b/bigframes/series.py
@@ -1598,6 +1598,10 @@ def __getattr__(self, key: str):
         else:
             raise AttributeError(key)
 
+    def __setitem__(self, key, value) -> None:
+        """Set item using direct assignment, delegating to .loc indexer."""
+        self.loc[key] = value
+
     def _apply_aggregation(
         self, op: agg_ops.UnaryAggregateOp | agg_ops.NullaryAggregateOp
     ) -> Any:
diff --git a/tests/system/small/test_series.py b/tests/system/small/test_series.py
index 6760d63a20..80802bab1c 100644
--- a/tests/system/small/test_series.py
+++ b/tests/system/small/test_series.py
@@ -510,6 +510,69 @@ def test_series___getitem___with_default_index(scalars_dfs):
     assert bf_result == pd_result
 
 
+@pytest.mark.parametrize(
+    ("index_col", "key", "value"),
+    (
+        ("int64_too", 2, "new_string_value"),
+        ("string_col", "Hello, World!", "updated_value"),
+        ("int64_too", 0, None),
+    ),
+)
+def test_series___setitem__(scalars_dfs, index_col, key, value):
+    col_name = "string_col"
+    scalars_df, scalars_pandas_df = scalars_dfs
+    scalars_df = scalars_df.set_index(index_col, drop=False)
+    scalars_pandas_df = scalars_pandas_df.set_index(index_col, drop=False)
+
+    bf_series = scalars_df[col_name]
+    pd_series = scalars_pandas_df[col_name].copy()
+
+    bf_series[key] = value
+    pd_series[key] = value
+
+    pd.testing.assert_series_equal(bf_series.to_pandas(), pd_series)
+
+
+@pytest.mark.parametrize(
+    ("key", "value"),
+    (
+        (-2, 999),
+        (-1, 888),
+        (0, None),
+        (1, 777),
+    ),
+)
+def test_series___setitem___with_int_key_numeric(scalars_dfs, key, value):
+    col_name = "int64_col"
+    index_col = "int64_too"
+    scalars_df, scalars_pandas_df = scalars_dfs
+    scalars_df = scalars_df.set_index(index_col, drop=False)
+    scalars_pandas_df = scalars_pandas_df.set_index(index_col, drop=False)
+
+    bf_series = scalars_df[col_name]
+    pd_series = scalars_pandas_df[col_name].copy()
+
+    bf_series[key] = value
+    pd_series[key] = value
+
+    pd.testing.assert_series_equal(bf_series.to_pandas(), pd_series)
+
+
+def test_series___setitem___with_default_index(scalars_dfs):
+    col_name = "float64_col"
+    key = 2
+    value = 123.456
+    scalars_df, scalars_pandas_df = scalars_dfs
+
+    bf_series = scalars_df[col_name]
+    pd_series = scalars_pandas_df[col_name].copy()
+
+    bf_series[key] = value
+    pd_series[key] = value
+
+    assert bf_series.to_pandas().iloc[key] == pd_series.iloc[key]
+
+
 @pytest.mark.parametrize(
     ("col_name",),
     (

From fa7e4d637f6bf2011d1000243280e53bb9be93f8 Mon Sep 17 00:00:00 2001
From: Shuowei Li <shuowei@google.com>
Date: Thu, 26 Jun 2025 04:31:05 +0000
Subject: [PATCH 2/2] testcase update

---
 tests/system/small/test_series.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tests/system/small/test_series.py b/tests/system/small/test_series.py
index 80802bab1c..d513b0e780 100644
--- a/tests/system/small/test_series.py
+++ b/tests/system/small/test_series.py
@@ -536,10 +536,10 @@ def test_series___setitem__(scalars_dfs, index_col, key, value):
 @pytest.mark.parametrize(
     ("key", "value"),
     (
-        (-2, 999),
-        (-1, 888),
+        (0, 999),
+        (1, 888),
         (0, None),
-        (1, 777),
+        (-2345, 777),
     ),
 )
 def test_series___setitem___with_int_key_numeric(scalars_dfs, key, value):