code sample for pandas-dev#21720

Author: simonjayhawkins

bisect/21720.py

@@ -0,0 +1,13 @@
+# BUG: df.duplicated treats None as np.nan in object columns #21720
+
+import numpy as np
+import pandas as pd
+
+print(pd.__version__)
+
+s = pd.Series([np.nan, 3, 3, None, np.nan], dtype=object)
+result = s.to_frame().duplicated()
+print(result)
+
+expected = pd.Series([0, 0, 1, 1, 1], dtype=bool)
+pd.testing.assert_series_equal(result, expected)