[data] use iloc to access pandas Series (ray-project#42890)

Signed-off-by: Hao Chen <[email protected]>

python/ray/data/_internal/pandas_block.py

@@ -58,6 +58,8 @@ class PandasRow(TableRow):
     def __getitem__(self, key: Union[str, List[str]]) -> Any:
         from ray.data.extensions import TensorArrayElement
 
+        pd = lazy_import_pandas()
+
         def get_item(keys: List[str]) -> Any:
             col = self._row[keys]
             if len(col) == 0:
@@ -67,12 +69,12 @@ def get_item(keys: List[str]) -> Any:
             if isinstance(items[0], TensorArrayElement):
                 # Getting an item in a Pandas tensor column may return
                 # a TensorArrayElement, which we have to convert to an ndarray.
-                return tuple([item.to_numpy() for item in items])
+                return pd.Series(item.to_numpy() for item in items)
 
             try:
                 # Try to interpret this as a numpy-type value.
                 # See https://stackoverflow.com/questions/9452775/converting-numpy-dtypes-to-native-python-types.  # noqa: E501
-                return tuple([item.as_py() for item in items])
+                return pd.Series(item.as_py() for item in items)
 
             except (AttributeError, ValueError):
                 # Fallback to the original form.
@@ -86,7 +88,7 @@ def get_item(keys: List[str]) -> Any:
         if items is None:
             return None
         elif is_single_item:
-            return items[0]
+            return items.iloc[0]
         else:
             return items