code sample for pandas-dev#42477

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