code sample for pandas-dev#43267

Author: simonjayhawkins

bisect/43267.py

@@ -0,0 +1,31 @@
+# BUG: crash in df.groupby.rolling.mean with forward window indexer #43267
+
+import numpy as np
+import pandas as pd
+
+print(pd.__version__)
+
+df = pd.DataFrame(
+    {"B": [np.nan, 1, 2, np.nan, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]}
+)
+df["A"] = 1
+indexer = pd.api.indexers.FixedForwardWindowIndexer(window_size=12)  # forward 12 period
+
+# First time is OK
+df["mean"] = (
+    df.groupby("A")["B"]
+    .rolling(window=indexer, min_periods=1)
+    .mean()
+    .reset_index()
+    .loc[:, "B"]
+)
+
+# Second time would crash!
+df["mean"] = (
+    df.groupby("A")["B"]
+    .rolling(window=indexer, min_periods=1)
+    .mean()
+    .reset_index()
+    .loc[:, "B"]
+)
+print(df)