API: Default ExtensionArray.astype

(cherry picked from commit 943a915562b72bed147c857de927afa0daf31c1a)

pandas/core/arrays/base.py

@@ -1,4 +1,6 @@
 """An interface for extending pandas with custom arrays."""
+import numpy as np
+
 from pandas.errors import AbstractMethodError
 
 _not_implemented_message = "{} does not implement {}."
@@ -138,6 +140,34 @@ def nbytes(self):
     # ------------------------------------------------------------------------
     # Additional Methods
     # ------------------------------------------------------------------------
+    def astype(self, dtype, copy=True):
+        """Cast to a NumPy array with 'dtype'.
+
+        The default implementation only allows casting to 'object' dtype.
+
+        Parameters
+        ----------
+        dtype : str or dtype
+            Typecode or data-type to which the array is cast.
+        copy : bool, default True
+            Whether to copy the data, even if not necessary. If False,
+            a copy is made only if the old dtype does not match the
+            new dtype.
+
+        Returns
+        -------
+        array : ndarray
+            NumPy ndarray with 'dtype' for its dtype.
+        """
+        np_dtype = np.dtype(dtype)
+
+        if np_dtype != 'object':
+            msg = ("{} can only be coerced to 'object' dtype, "
+                   "not '{}'.").format(type(self).__name__, dtype)
+            raise ValueError(msg)
+
+        return np.array(self, dtype=np_dtype, copy=copy)
+
     def isna(self):
         # type: () -> np.ndarray
         """Boolean NumPy array indicating if each value is missing.

pandas/tests/extension_arrays/test_common.py

@@ -0,0 +1,34 @@
+import numpy as np
+
+import pandas.util.testing as tm
+from pandas.core.arrays import ExtensionArray
+
+
+class DummyArray(ExtensionArray):
+
+    def __init__(self, data):
+        self.data = data
+
+    def __array__(self, dtype):
+        return self.data
+
+
+def test_astype():
+    arr = DummyArray(np.array([1, 2, 3]))
+    expected = np.array([1, 2, 3], dtype=object)
+
+    result = arr.astype(object)
+    tm.assert_numpy_array_equal(result, expected)
+
+    result = arr.astype('object')
+    tm.assert_numpy_array_equal(result, expected)
+
+
+def test_astype_raises():
+    arr = DummyArray(np.array([1, 2, 3]))
+
+    xpr = ("DummyArray can only be coerced to 'object' dtype, not "
+           "'<class 'int'>'")
+
+    with tm.assert_raises_regex(ValueError, xpr):
+        arr.astype(int)