Merge 3.6 (issue #26796)

Doc/library/asyncio-eventloop.rst

@@ -671,6 +671,13 @@ pool of processes). By default, an event loop uses a thread pool executor
 
    This method is a :ref:`coroutine <coroutine>`.
 
+   .. versionchanged:: 3.5.3
+      :meth:`BaseEventLoop.run_in_executor` no longer configures the
+      ``max_workers`` of the thread pool executor it creates, instead
+      leaving it up to the thread pool executor
+      (:class:`~concurrent.futures.ThreadPoolExecutor`) to set the
+      default.
+
 .. method:: AbstractEventLoop.set_default_executor(executor)
 
    Set the default executor used by :meth:`run_in_executor`.

Lib/asyncio/base_events.py

@@ -41,9 +41,6 @@ from .log import logger
 __all__ = ['BaseEventLoop']
 
 
-# Argument for default thread pool executor creation.
-_MAX_WORKERS = 5
-
 # Minimum number of _scheduled timer handles before cleanup of
 # cancelled handles is performed.
 _MIN_SCHEDULED_TIMER_HANDLES = 100
@@ -620,7 +617,7 @@ class BaseEventLoop(events.AbstractEventLoop):
         if executor is None:
             executor = self._default_executor
             if executor is None:
-                executor = concurrent.futures.ThreadPoolExecutor(_MAX_WORKERS)
+                executor = concurrent.futures.ThreadPoolExecutor()
                 self._default_executor = executor
         return futures.wrap_future(executor.submit(func, *args), loop=self)