Closes #21527: Add default number of workers to ThreadPoolExecutor. (Claudiu Popa.)
diff --git a/Doc/library/concurrent.futures.rst b/Doc/library/concurrent.futures.rst
index 08c926a..e487817 100644
--- a/Doc/library/concurrent.futures.rst
+++ b/Doc/library/concurrent.futures.rst
@@ -115,11 +115,19 @@
executor.submit(wait_on_future)
-.. class:: ThreadPoolExecutor(max_workers)
+.. class:: ThreadPoolExecutor(max_workers=None)
An :class:`Executor` subclass that uses a pool of at most *max_workers*
threads to execute calls asynchronously.
+ .. versionchanged:: 3.5
+ If *max_workers* is ``None`` or
+ not given, it will default to the number of processors on the machine,
+ multiplied by ``5``, assuming that :class:`ThreadPoolExecutor` is often
+ used to overlap I/O instead of CPU work and the number of workers
+ should be higher than the number of workers
+ for :class:`ProcessPoolExecutor`.
+
.. _threadpoolexecutor-example: