fix: for reset_index on unnamed multiindex, always use level_[n] label (#182)

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Author: milkshakeiii

bigframes/core/blocks.py

@@ -282,7 +282,7 @@ def reset_index(self, drop: bool = True) -> Block:
  column_labels_modified = self.column_labels
  for level, label in enumerate(index_labels):
  if label is None:
- if "index" not in self.column_labels:
+ if "index" not in self.column_labels and len(index_labels) <= 1:
  label = "index"
  else:
  label = f"level_{level}"

tests/system/small/test_dataframe.py

@@ -1235,6 +1235,28 @@ def test_reset_index_with_unnamed_index(
  pandas.testing.assert_frame_equal(bf_result, pd_result)
 
 
+def test_reset_index_with_unnamed_multiindex(
+ scalars_df_index,
+ scalars_pandas_df_index,
+):
+ bf_df = dataframe.DataFrame(
+ ([1, 2, 3], [2, 5, 7]),
+ index=pd.MultiIndex.from_tuples([("a", "aa"), ("a", "aa")]),
+ )
+ pd_df = pd.DataFrame(
+ ([1, 2, 3], [2, 5, 7]),
+ index=pd.MultiIndex.from_tuples([("a", "aa"), ("a", "aa")]),
+ )
+
+ bf_df = bf_df.reset_index()
+ pd_df = pd_df.reset_index()
+
+ assert pd_df.columns[0] == "level_0"
+ assert bf_df.columns[0] == "level_0"
+ assert pd_df.columns[1] == "level_1"
+ assert bf_df.columns[1] == "level_1"
+
+
 def test_reset_index_with_unnamed_index_and_index_column(
  scalars_df_index,
  scalars_pandas_df_index,