DOC: Various EA docs

Closes pandas-dev#20668

Author: TomAugspurger

doc/source/extending.rst

@@ -57,6 +57,13 @@ If you write a custom accessor, make a pull request adding it to our
 Extension Types
 ---------------
 
+.. versionadded:: 0.23.0
+
+.. warning::
+
+   The ``ExtensionDtype`` and ``ExtensionArray`` APIs are new and
+   experimental. They may change between versions without warning.
+
 Pandas defines an interface for implementing data types and arrays that *extend*
 NumPy's type system. Pandas itself uses the extension system for some types
 that aren't built into NumPy (categorical, period, interval, datetime with
@@ -106,6 +113,24 @@ by some other storage type, like Python lists.
 See the `extension array source`_ for the interface definition. The docstrings
 and comments contain guidance for properly implementing the interface.
 
+We provide a test suite for ensuring that your extension arrays satisfy the expected
+behavior. To use the test-suite, you must provide several pytest fixtures and inherit
+from the base test class. The required fixtures are found in
+https://github.com/pandas-dev/pandas/blob/master/pandas/tests/extension/conftest.py.
+
+To use a test, subclass it
+
+.. code-block:: python
+
+   from pandas.tests.extension import base
+
+   class TestConstructors(base.BaseConstructorsTests):
+       pass
+
+
+See https://github.com/pandas-dev/pandas/blob/master/pandas/tests/extension/base/__init__.py
+for a list of all the tests available.
+
 .. _extension dtype source: https://github.com/pandas-dev/pandas/blob/master/pandas/core/dtypes/base.py
 .. _extension array source: https://github.com/pandas-dev/pandas/blob/master/pandas/core/arrays/base.py
 

doc/source/install.rst

@@ -15,7 +15,7 @@ Instructions for installing from source,
 `PyPI <http://pypi.python.org/pypi/pandas>`__, `ActivePython <https://www.activestate.com/activepython/downloads>`__, various Linux distributions, or a
 `development version <http://github.com/pandas-dev/pandas>`__ are also provided.
 
-.. _install.dropping_27:
+.. _install.dropping-27:
 
 Plan for dropping Python 2.7
 ----------------------------

doc/source/whatsnew/v0.23.0.txt

@@ -11,7 +11,7 @@ version.
 .. warning::
 
    Starting January 1, 2019, pandas feature releases will support Python 3 only.
-   See :ref:`here <install.dropping_27>` for more.
+   See :ref:`install.dropping-27` for more.
 
 .. _whatsnew_0230.enhancements:
 
@@ -306,8 +306,8 @@ Supplying a ``CategoricalDtype`` will make the categories in each column consist
 
 .. _whatsnew_023.enhancements.extension:
 
-Extending Pandas with Custom Types
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Extending Pandas with Custom Types (Experimental)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Pandas now supports storing array-like objects that aren't necessarily 1-D NumPy
 arrays as columns in a DataFrame or values in a Series. This allows third-party

pandas/core/arrays/base.py

@@ -14,12 +14,15 @@ class ExtensionArray(object):
     with a custom type and will not attempt to coerce them to objects. They
     may be stored directly inside a :class:`DataFrame` or :class:`Series`.
 
+    .. versionadded:: 0.23.0
+
     Notes
     -----
     The interface includes the following abstract methods that must be
     implemented by subclasses:
 
     * _constructor_from_sequence
+    * _from_factorized
     * __getitem__
     * __len__
     * dtype
@@ -35,6 +38,15 @@ class ExtensionArray(object):
     * _can_hold_na
     * _formatting_values
 
+    Some methods require casting the ExtensionArray to an ndarray of Python
+    objects, which may be expensive. When performance is a concern, we highly
+    recommend overriding the following methods.
+
+    * fillna
+    * unique
+    * factorize / _values_for_factorize
+    * argsort / _values_for_argsort
+
     This class does not inherit from 'abc.ABCMeta' for performance reasons.
     Methods and properties required by the interface raise
     ``pandas.errors.AbstractMethodError`` and no ``register`` method is
@@ -50,10 +62,6 @@ class ExtensionArray(object):
     by some other storage type, like Python lists. Pandas makes no
     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.
     """
     # '_typ' is for pandas.core.dtypes.generic.ABCExtensionArray.
     # Don't override this.