feat: support item assignment in series (#1859)

* Add item assignment for bigframes dataframe

* testcase update

Author: shuoweil

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:

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"),
+    (
+        (0, 999),
+        (1, 888),
+        (0, None),
+        (-2345, 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",),
     (