chore(bigframes): add sql property to BF series (googleapis#16858)

This makes it easier to check the SQL expression for BF series.

Plus, fixed a format of an unrelated file.

Author: sycai

packages/bigframes/bigframes/bigquery/_operations/ml.py

@@ -20,12 +20,12 @@
 import google.cloud.bigquery
 import pandas as pd
 
+import bigframes.core.col as col
 import bigframes.core.logging.log_adapter as log_adapter
 import bigframes.core.sql.ml
 import bigframes.dataframe as dataframe
 import bigframes.ml.base
 import bigframes.session
-import bigframes.core.col as col
 from bigframes.bigquery._operations import utils
 
 

packages/bigframes/bigframes/series.py

@@ -353,6 +353,16 @@ def _struct_fields(self) -> List[str]:
         struct_type = typing.cast(pa.StructType, self._dtype.pyarrow_dtype)
         return [struct_type.field(i).name for i in range(struct_type.num_fields)]
 
+    @property
+    def sql(self) -> str:
+        """Compiles this Series's expression tree to SQL.
+
+        Returns:
+            A string representing the compiled SQL.
+        """
+
+        return self.to_frame().sql
+
     @validations.requires_ordering()
     def transpose(self) -> Series:
         return self

packages/bigframes/tests/system/small/test_series.py

@@ -4999,3 +4999,9 @@ def test_series_item_with_empty(session):
 
     with pytest.raises(ValueError, match=re.escape(expected_message)):
         bf_s_empty.item()
+
+
+def test_series_sql(session):
+    s = bigframes.pandas.Series([], session=session)
+
+    assert len(s.sql) > 0