TST: groupby-reindex on DTI (pandas-dev#33638)

Author: CloseChoice

pandas/tests/groupby/test_apply.py

@@ -491,6 +491,26 @@ def test_apply_with_duplicated_non_sorted_axis(test_series):
         tm.assert_frame_equal(result, expected)
 
 
+def test_apply_reindex_values():
+    # GH: 26209
+    # reindexing from a single column of a groupby object with duplicate indices caused
+    # a ValueError (cannot reindex from duplicate axis) in 0.24.2, the problem was
+    # solved in #30679
+    values = [1, 2, 3, 4]
+    indices = [1, 1, 2, 2]
+    df = pd.DataFrame(
+        {"group": ["Group1", "Group2"] * 2, "value": values}, index=indices
+    )
+    expected = pd.Series(values, index=indices, name="value")
+
+    def reindex_helper(x):
+        return x.reindex(np.arange(x.index.min(), x.index.max() + 1))
+
+    # the following group by raised a ValueError
+    result = df.groupby("group").value.apply(reindex_helper)
+    tm.assert_series_equal(expected, result)
+
+
 def test_apply_corner_cases():
     # #535, can't use sliding iterator