fix: fix read_gbq_function issue in dataframe apply method

bigframes/dataframe.py

@@ -3958,8 +3958,26 @@ def apply(self, func, *, axis=0, args: typing.Tuple = (), **kwargs):
  result_series.name = None
  return result_series
 
+ # This is when the func as a remote function is applied to each element of
+ # the dataframe (not supported).
+ # In Bigframes remote function, DataFrame '.apply' method is specifically
+ # designed to work with row-wise or column-wise operations, where the input
+ # to the applied function should be a Series, not a scalar.
+ if hasattr(func, "is_row_processor") and not func.is_row_processor:
+ raise NotImplementedError(
+ "In Bigframes remote function, DataFrame '.apply()' does not "
+ "support element-wise application. Please use '.map()' instead."
+ )
+
+ if hasattr(func, "bigframes_remote_function") and (args or kwargs):
+ warnings.warn(
+ "The args and kwargs are not supported in the remote function.",
+ category=bigframes.exceptions.ArgsAndKwargsNotSupportedWarning,
+ )
+
  # Per-column apply
  results = {name: func(col, *args, **kwargs) for name, col in self.items()}
+
  if all(
  [
  isinstance(val, bigframes.series.Series) or utils.is_list_like(val)

bigframes/exceptions.py

@@ -81,3 +81,7 @@ class ApiDeprecationWarning(FutureWarning):
 
 class BadIndexerKeyWarning(Warning):
  """The indexer key is not used correctly."""
+
+
+class ArgsAndKwargsNotSupportedWarning(Warning):
+ """The args and kwargs are not supported."""

tests/system/small/test_remote_function.py

@@ -839,6 +839,28 @@ def test_read_gbq_function_enforces_explicit_types(
  )
 
 
+@pytest.mark.flaky(retries=2, delay=120)
+def test_df_apply_scalar_func(session, scalars_dfs):
+ scalars_df, _ = scalars_dfs
+ bdf = bigframes.pandas.DataFrame(
+ {
+ "Column1": scalars_df["string_col"],
+ "Column2": scalars_df["string_col"],
+ }
+ )
+
+ # The "cw_lower_case_ascii_only" is a scalar function.
+ func_ref = session.read_gbq_function("bqutil.fn.cw_lower_case_ascii_only")
+
+ # DataFrame '.apply()' only supports series level application.
+ with pytest.raises(NotImplementedError) as context:
+ bdf.apply(func_ref)
+ assert str(context.value) == (
+ "In Bigframes remote function, DataFrame '.apply()' does not support "
+ "element-wise application. Please use '.map()' instead."
+ )
+
+
 @pytest.mark.flaky(retries=2, delay=120)
 def test_read_gbq_function_multiple_inputs_not_a_row_processor(session):
  with pytest.raises(ValueError) as context: