bpo-17140: Document multiprocessing's ThreadPool (GH-23812)
Up until now, the `multiprocessing.pool.ThreadPool` class has gone
undocumented, despite being a public class in multiprocessing that is
included in `multiprocessing.pool.__all__`.
diff --git a/Doc/library/multiprocessing.rst b/Doc/library/multiprocessing.rst
index 128aa43..e109b1e 100644
--- a/Doc/library/multiprocessing.rst
+++ b/Doc/library/multiprocessing.rst
@@ -2661,6 +2661,46 @@
:mod:`multiprocessing.dummy` replicates the API of :mod:`multiprocessing` but is
no more than a wrapper around the :mod:`threading` module.
+.. currentmodule:: multiprocessing.pool
+
+In particular, the ``Pool`` function provided by :mod:`multiprocessing.dummy`
+returns an instance of :class:`ThreadPool`, which is a subclass of
+:class:`Pool` that supports all the same method calls but uses a pool of
+worker threads rather than worker processes.
+
+
+.. class:: ThreadPool([processes[, initializer[, initargs]]])
+
+ A thread pool object which controls a pool of worker threads to which jobs
+ can be submitted. :class:`ThreadPool` instances are fully interface
+ compatible with :class:`Pool` instances, and their resources must also be
+ properly managed, either by using the pool as a context manager or by
+ calling :meth:`~multiprocessing.pool.Pool.close` and
+ :meth:`~multiprocessing.pool.Pool.terminate` manually.
+
+ *processes* is the number of worker threads to use. If *processes* is
+ ``None`` then the number returned by :func:`os.cpu_count` is used.
+
+ If *initializer* is not ``None`` then each worker process will call
+ ``initializer(*initargs)`` when it starts.
+
+ Unlike :class:`Pool`, *maxtasksperchild* and *context* cannot be provided.
+
+ .. note::
+
+ A :class:`ThreadPool` shares the same interface as :class:`Pool`, which
+ is designed around a pool of processes and predates the introduction of
+ the :class:`concurrent.futures` module. As such, it inherits some
+ operations that don't make sense for a pool backed by threads, and it
+ has its own type for representing the status of asynchronous jobs,
+ :class:`AsyncResult`, that is not understood by any other libraries.
+
+ Users should generally prefer to use
+ :class:`concurrent.futures.ThreadPoolExecutor`, which has a simpler
+ interface that was designed around threads from the start, and which
+ returns :class:`concurrent.futures.Future` instances that are
+ compatible with many other libraries, including :mod:`asyncio`.
+
.. _multiprocessing-programming: