Added _constructor

pandas/core/arrays/base.py

@@ -27,11 +27,11 @@ class ExtensionArray(object):
     * copy
     * _concat_same_type
 
-    Some additional methods are available to satisfy pandas' internal, private
-    block API.
+    Some additional methods are available to satisfy pandas' internals.
 
     * _can_hold_na
     * _formatting_values
+    * _constructor
 
     This class does not inherit from 'abc.ABCMeta' for performance reasons.
     Methods and properties required by the interface raise
@@ -49,13 +49,32 @@ class ExtensionArray(object):
     assumptions on how the data are stored, just that it can be converted
     to a NumPy array.
 
-    Extension arrays should be able to be constructed with instances of
-    the class, i.e. ``ExtensionArray(extension_array)`` should return
-    an instance, not error.
+    There are a few restrictions on how ExtensionArrays are created.
+
+    * An ExtensionArray should be valid, i.e.
+      ``ExtensionArray(extension_array)`` should return an instance
+    * A sequence of the scalar type should be valid, i.e.
+      ``ExtensionArray(Sequence[ExtensionDtype.type]])`` should return
+      an instance, not error.
     """
     # '_typ' is for pandas.core.dtypes.generic.ABCExtensionArray.
     # Don't override this.
     _typ = 'extension'
+
+    @classmethod
+    def _constructor(cls, data):
+        """Construct a new instance of of the extension array.
+
+        Parameters
+        ----------
+        data : Sequence
+
+        Returns
+        -------
+        ExtensionArray
+        """
+        return cls(data)
+
     # ------------------------------------------------------------------------
     # Must be a Sequence
     # ------------------------------------------------------------------------
@@ -226,7 +245,7 @@ def unique(self):
         from pandas import unique
 
         uniques = unique(self.astype(object))
-        return type(self)(uniques)
+        return self._constructor(uniques)
 
     # ------------------------------------------------------------------------
     # Indexing methods