Clarify type, kind, init

pandas/core/arrays/base.py

@@ -20,9 +20,6 @@ class ExtensionArray(object):
 
     **Restrictions on your class constructor**
 
-        * Your class should be able to be constructed with no arguments,
-          i.e. ``ExtensionArray()`` returns an instance.
-          TODO: See comment in ``ExtensionDtype.construct_from_string``
         * Your class should be able to be constructed with instances of
           our class, i.e. ``ExtensionArray(extension_array)`` should returns
           an instance.

pandas/core/dtypes/base.py

@@ -12,15 +12,21 @@ class ExtensionDtype(object):
     @abc.abstractmethod
     def type(self):
         # type: () -> type
-        """The scalar type for your array, e.g. ``int`` or ``object``."""
+        """The scalar type for your array, e.g. ``int``
+
+        It's expected ``ExtensionArray[item]`` returns an instance
+        of ``ExtensionDtype.type`` for scalar ``item``.
+        """
 
     @property
     def kind(self):
         # type () -> str
         """A character code (one of 'biufcmMOSUV'), default 'O'
 
         This should match the NumPy dtype used when your array is
-        converted to an ndarray, which is probably 'O' for object.
+        converted to an ndarray, which is probably 'O' for object if
+        your extension type cannot be represented as a built-in NumPy
+        type.
 
         See Also
         --------