feat: embed str values directly in pyformat

Updates `bigframes/core/pyformat.py` to directly embed string values in the formatted output. This allows for dynamic substitution of SQL identifiers, such as table or column names.

Updated unit tests to reflect this new behavior by using valid SQL identifiers in the test cases.

Author: google-labs-jules[bot]

bigframes/core/pyformat.py

@@ -104,6 +104,9 @@ def _field_to_template_value(
     if isinstance(value, bigframes.dataframe.DataFrame):
         return _table_to_sql(value._to_placeholder_table(dry_run=dry_run))
 
+    if isinstance(value, str):
+        return value
+
     return bigframes.core.sql.simple_literal(value)
 
 

tests/unit/core/test_pyformat.py

@@ -444,7 +444,7 @@ def test_pyformat_with_pandas_dataframe_not_dry_run_no_session_raises_valueerror
 
 def test_pyformat_with_query_string_replaces_variables(session):
     pyformat_args = {
-        "my_string": "some string value",
+        "my_string": "`my_table`",
         "max_value": 2.25,
         "year": 2025,
         "null_value": None,
@@ -456,19 +456,17 @@ def test_pyformat_with_query_string_replaces_variables(session):
     SELECT {year} - year  AS age,
     @myparam AS myparam,
     '{{my_string}}' AS escaped_string,
-    {my_string} AS my_string,
-    {null_value} AS null_value,
-    FROM my_dataset.my_table
+    *
+    FROM {my_string}
     WHERE height < {max_value}
     """.strip()
 
     expected_sql = """
     SELECT 2025 - year  AS age,
     @myparam AS myparam,
     '{my_string}' AS escaped_string,
-    'some string value' AS my_string,
-    NULL AS null_value,
-    FROM my_dataset.my_table
+    *
+    FROM `my_table`
     WHERE height < 2.25
     """.strip()
 

tests/unit/session/test_read_gbq_colab.py

@@ -60,7 +60,7 @@ def test_read_gbq_colab_includes_formatted_values_in_dry_run(monkeypatch, dry_ru
 
     pyformat_args = {
         "some_integer": 123,
-        "some_string": "This could be dangerous, but we escape it",
+        "some_string": "some_column",
         "bf_df": bf_df,
         "pd_df": pd_df,
         # This is not a supported type, but ignored if not referenced.
@@ -84,7 +84,7 @@ def test_read_gbq_colab_includes_formatted_values_in_dry_run(monkeypatch, dry_ru
     expected = textwrap.dedent(
         f"""
         SELECT 123 as some_integer,
-        'This could be dangerous, but we escape it' as some_string,
+        some_column as some_string,
         '{{escaped}}' as escaped
         FROM `proj`.`dset`.`temp_{"table" if dry_run else "view"}` AS bf_df
         FULL OUTER JOIN `proj`.`dset`.`temp_{"table" if dry_run else "view"}` AS pd_df