From a7e15c0af2e8fe5ac9c00bddb0a9111d5acdefde Mon Sep 17 00:00:00 2001
From: Simon Hawkins <simonjayhawkins@gmail.com>
Date: Fri, 16 Jul 2021 12:45:28 +0100
Subject: [PATCH] code sample for #42477

---
 bisect/42477.py | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)
 create mode 100644 bisect/42477.py

diff --git a/bisect/42477.py b/bisect/42477.py
new file mode 100644
index 0000000000000..fad31059dd294
--- /dev/null
+++ b/bisect/42477.py
@@ -0,0 +1,21 @@
+# 1.3.0 PerformanceWarning: DataFrame is highly fragmented. #42477
+
+import numpy as np
+import pandas as pd
+
+print(pd.__version__)
+
+df = pd.DataFrame(
+    {"a": np.random.randint(0, 100, size=55), "b": np.random.randint(0, 100, size=55)}
+)
+
+# Assign > 100 new columns to the dataframe
+for i in range(0, 100):
+    df.loc[:, f"n_{i}"] = np.random.randint(0, 100, size=55)
+    # Alternative assignment - triggers Performancewarnings here already.
+    # df[f'n_{i}'] = np.random.randint(0, 100, size=55)
+print(df._data.nblocks)
+
+df1 = df.copy()
+result = df1._data.nblocks
+assert result == 1, result