gh-151950: Document asyncio.BaseEventLoop and fix asyncio-extending.rst (#151976)

* gh-151950: Document `async.BaseEventLoop` and fix `asyncio-extending.rst`

* Apply suggestions from code review

Co-authored-by: Stan Ulbrych <stan@python.org>

* Update Doc/library/asyncio-extending.rst

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

---------

Co-authored-by: Stan Ulbrych <stan@python.org>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>

Author: 4 people

Doc/library/asyncio-eventloop.rst

@@ -1967,6 +1967,17 @@ By default asyncio is configured to use :class:`EventLoop`.
    methods that an alternative implementation of ``AbstractEventLoop``
    should have defined.
 
+.. class:: BaseEventLoop
+
+   A base class that partially implements :class:`AbstractEventLoop`
+   to simplify writing custom event loops.
+
+   It provides default implementations of many :class:`AbstractEventLoop`
+   methods, so subclasses have fewer methods to implement themselves.
+   In return, a subclass is expected to implement a number of internal,
+   protected methods.
+   See :ref:`writing_custom_event_loop` for more information.
+
 
 Examples
 ========

Doc/library/asyncio-extending.rst

@@ -12,14 +12,17 @@ classes. Asyncio has helpers that could be used to simplify this task.
 
    Third-parties should reuse existing asyncio code with caution,
    a new Python version is free to break backward compatibility
-   in *internal* part of API.
+   in the *internal* part of the API.
 
 
-Writing a Custom Event Loop
+.. _writing_custom_event_loop:
+
+Writing a custom event loop
 ===========================
 
-:class:`asyncio.AbstractEventLoop` declares very many methods.  Implementing all them
-from scratch is a tedious job.
+:class:`asyncio.AbstractEventLoop` declares very many methods.
+See :ref:`asyncio-event-loop-methods` for the full list.
+Implementing them all from scratch is a tedious job.
 
 A loop can get many common methods implementation for free by inheriting from
 :class:`asyncio.BaseEventLoop`.

Doc/tools/.nitignore

@@ -3,7 +3,6 @@
 # Keep lines sorted lexicographically to help avoid merge conflicts.
 
 Doc/library/ast.rst
-Doc/library/asyncio-extending.rst
 Doc/library/logging.config.rst
 Doc/library/logging.handlers.rst
 Doc/library/multiprocessing.rst