fix: Improve Series.replace for dict input (#907)

Author: shobsi

bigframes/series.py

@@ -581,6 +581,9 @@ def _simple_replace(self, to_replace_list: typing.Sequence, value):
         return Series(block.select_column(result_col))
 
     def _mapping_replace(self, mapping: dict[typing.Hashable, typing.Hashable]):
+        if not mapping:
+            return self.copy()
+
         tuples = []
         lcd_types: list[typing.Optional[bigframes.dtypes.Dtype]] = []
         for key, value in mapping.items():
@@ -597,6 +600,7 @@ def _mapping_replace(self, mapping: dict[typing.Hashable, typing.Hashable]):
         result_dtype = functools.reduce(
             lambda t1, t2: bigframes.dtypes.lcd_type(t1, t2) if (t1 and t2) else None,
             lcd_types,
+            self.dtype,
         )
         if not result_dtype:
             raise NotImplementedError(
@@ -605,7 +609,9 @@ def _mapping_replace(self, mapping: dict[typing.Hashable, typing.Hashable]):
         block, result = self._block.apply_unary_op(
             self._value_column, ops.MapOp(tuple(tuples))
         )
-        return Series(block.select_column(result))
+        replaced = Series(block.select_column(result))
+        replaced.name = self.name
+        return replaced
 
     @validations.requires_ordering()
     @validations.requires_index

tests/system/small/test_series.py

@@ -458,6 +458,29 @@ def test_series_replace_list_scalar(scalars_dfs):
     )
 
 
+@pytest.mark.parametrize(
+    ("replacement_dict",),
+    (
+        ({"Hello, World!": "Howdy, Planet!", "T": "R"},),
+        ({},),
+    ),
+    ids=[
+        "non-empty",
+        "empty",
+    ],
+)
+def test_series_replace_dict(scalars_dfs, replacement_dict):
+    scalars_df, scalars_pandas_df = scalars_dfs
+    col_name = "string_col"
+    bf_result = scalars_df[col_name].replace(replacement_dict).to_pandas()
+    pd_result = scalars_pandas_df[col_name].replace(replacement_dict)
+
+    pd.testing.assert_series_equal(
+        pd_result,
+        bf_result,
+    )
+
+
 @pytest.mark.parametrize(
     ("method",),
     (