TST: groupby apply called multiple times

pandas/tests/groupby/test_apply.py

@@ -974,3 +974,24 @@ def test_apply_function_with_indexing_return_column():
     result = df.groupby("foo1", as_index=False).apply(lambda x: x.mean())
     expected = DataFrame({"foo1": ["one", "three", "two"], "foo2": [3.0, 4.0, 4.0]})
     tm.assert_frame_equal(result, expected)
+
+
+def test_apply_function_called_count(capsys):
+    # GH: 31111
+    # groupby-apply need to execute len(set(group_by_columns)) times
+
+    expected = 2  # Number of times `apply` should call a function for the current test
+
+    df = pd.DataFrame(
+        {
+            "group_by_column": [0, 0, 0, 0, 1, 1, 1, 1],
+            "test_column": ["0", "2", "4", "6", "8", "10", "12", "14"],
+        },
+        index=["0", "2", "4", "6", "8", "10", "12", "14"],
+    )
+
+    df.groupby("group_by_column").apply(lambda df: print("function_called"))
+
+    result = capsys.readouterr().out.count("function_called")
+    # If `groupby` behaves unexpectedly, this test will break
+    assert result == expected