Use correct index when returning Series from GroupBy.apply() (#9016)

Closes #8898 

Originally, when returning a Series from a `GroupBy.apply()` operation, we would pass in `self.grouping.keys[offsets[:-1]]` as the index, which was meant to grab each unique group key, assuming that `self.grouping.keys` is sorted. However, because it is not sorted, this just ends up grabbing 5 group keys at random.

Since we are already calling `GroupBy._grouped()` in this operation, we can use the `group_names` returned by that as the index instead, which is what the result of `self.grouping.keys[offsets[:-1]]` would be if `self.grouping.keys` was sorted.

Authors:
  - Charles Blackmon-Luca (https://github.com/charlesbluca)

Approvers:
  - Michael Wang (https://github.com/isVoid)

URL: #9016

python/cudf/cudf/core/groupby/groupby.py

@@ -449,7 +449,7 @@ def mult(df):
         """
         if not callable(function):
             raise TypeError(f"type {type(function)} is not callable")
-        _, offsets, _, grouped_values = self._grouped()
+        group_names, offsets, _, grouped_values = self._grouped()
 
         ngroups = len(offsets) - 1
         if ngroups > self._MAX_GROUPS_BEFORE_WARN:
@@ -467,9 +467,7 @@ def mult(df):
             return self.obj.__class__()
 
         if cudf.utils.dtypes.is_scalar(chunk_results[0]):
-            result = cudf.Series(
-                chunk_results, index=self.grouping.keys[offsets[:-1]]
-            )
+            result = cudf.Series(chunk_results, index=group_names)
             result.index.names = self.grouping.names
         elif isinstance(chunk_results[0], cudf.Series):
             result = cudf.concat(chunk_results, axis=1).T

python/cudf/cudf/tests/test_groupby.py

@@ -2106,11 +2106,11 @@ def test_groupby_first(data, agg):
     assert_groupby_results_equal(expect, got, check_dtype=False)
 
 
-def test_groupby_apply_series_name():
+def test_groupby_apply_series():
     def foo(x):
         return x.sum()
 
-    got = make_frame(DataFrame, 3).groupby("x").y.apply(foo)
-    expect = make_frame(pd.DataFrame, 3).groupby("x").y.apply(foo)
+    got = make_frame(DataFrame, 100).groupby("x").y.apply(foo)
+    expect = make_frame(pd.DataFrame, 100).groupby("x").y.apply(foo)
 
-    assert expect.name == got.name
+    assert_groupby_results_equal(expect, got)