code sample for pandas-dev#42501

Author: simonjayhawkins

bisect/42501.py

@@ -0,0 +1,29 @@
+# BUG: extraneous copy of extension arrays in v1.3.0 #42501
+
+import pandas as pd
+from pandas.core.arrays.integer import coerce_to_array
+
+print(pd.__version__)
+
+import pandas as pd
+
+
+class IntegerArrayNoCopy(pd.core.arrays.IntegerArray):
+    @classmethod
+    def _from_sequence(cls, scalars, *, dtype=None, copy=False):
+        values, mask = coerce_to_array(scalars, dtype=dtype, copy=copy)
+        return IntegerArrayNoCopy(values, mask)
+
+    def copy(self):
+        raise NotImplementedError
+
+
+class Int16DtypeNoCopy(pd.Int16Dtype):
+    @classmethod
+    def construct_array_type(cls):
+        return IntegerArrayNoCopy
+
+
+df = pd.DataFrame({"col": [1, 4, None, 5]}, dtype=object)
+result = df.astype({"col": Int16DtypeNoCopy()}, copy=False)
+print(result)