add code sample for pandas-dev#41999

Author: simonjayhawkins

bisect/41999.py

@@ -0,0 +1,31 @@
+# BUG?: 1.3 behavior change with groupby, apply and side effect #41999
+
+import pandas as pd
+
+print(pd.__version__)
+
+
+def apply_func(df):
+    del df["MEMBER_ID"]
+
+    return df.head(1)
+
+
+df = pd.DataFrame.from_dict(
+    {
+        "MONTH": {0: "April", 1: "April", 2: "April"},
+        "MEMBER_ID": {0: "Member A", 1: "Member A", 2: "Member B"},
+        "ACTIVITY_CATEGORY": {
+            0: "Activity 1",
+            1: "Days off at homebase",
+            2: "Activity 1",
+        },
+        "FTE": {0: 1.0, 1: 1.0, 2: 0.75},
+        "FTE_OFF_DAYS": {0: 5, 1: 5, 2: 10},
+    }
+)
+grp = df.groupby(["MONTH", "FTE"], observed=True)[
+    ["MEMBER_ID", "FTE_OFF_DAYS", "ACTIVITY_CATEGORY"]
+]
+result = grp.apply(apply_func)
+print(result)