[REVIEW] Fix groupby apply for operations that fail on empty groups

CHANGELOG.md

@@ -84,6 +84,7 @@
 - PR #4756 Fix regex error checking for valid quantifier condition
 - PR #4777 Fix data pointer for column slices of zero length
 - PR #4770 Fix readonly flag in `Column. __cuda_array_interface__`
+- PR #4796 Fix groupby apply for operations that fail on empty groups
 
 
 # cuDF 0.13.0 (31 Mar 2020)

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

@@ -2,7 +2,6 @@
 
 import collections
 import functools
-import itertools
 import pickle
 
 import pandas as pd
@@ -327,8 +326,9 @@ def mult(df):
         if not callable(function):
             raise TypeError("type {!r} is not callable", type(function))
         _, offsets, grouped_values = self._grouped()
-        ends = itertools.chain(offsets[1:], [None])
-        chunks = [grouped_values[s:e] for s, e in zip(offsets, ends)]
+        chunks = [
+            grouped_values[s:e] for s, e in zip(offsets[:-1], offsets[1:])
+        ]
         result = cudf.concat([function(chk) for chk in chunks])
         if self._sort:
             result = result.sort_index()

python/cudf/cudf/tests/test_groupby.py

@@ -1078,3 +1078,18 @@ def test_groupby_agg_combinations(agg):
     assert_eq(
         pdf.groupby("a").agg(agg), gdf.groupby("a").agg(agg), check_dtype=False
     )
+
+
+def test_groupby_apply_noempty_group():
+    pdf = pd.DataFrame(
+        {"a": [1, 1, 2, 2], "b": [1, 2, 1, 2], "c": [1, 2, 3, 4]}
+    )
+    gdf = cudf.from_pandas(pdf)
+    assert_eq(
+        pdf.groupby("a")
+        .apply(lambda x: x.iloc[[0, 1]])
+        .reset_index(drop=True),
+        gdf.groupby("a")
+        .apply(lambda x: x.iloc[[0, 1]])
+        .reset_index(drop=True),
+    )