bpo-34075: Deprecate non-ThreadPoolExecutor in loop.set_default_executor() (GH-8533)

Various asyncio internals expect that the default executor is a
`ThreadPoolExecutor`, so deprecate passing anything else to
`loop.set_default_executor()`.

2 files changed, 14 insertions, 1 deletions

diff --git a/Doc/library/asyncio-eventloop.rst b/Doc/library/asyncio-eventloop.rst
index 317f3fb..cf7b6d8 100644
--- a/Doc/library/asyncio-eventloop.rst
+++ b/Doc/library/asyncio-eventloop.rst
@@ -906,7 +906,14 @@ pool of processes). By default, an event loop uses a thread pool executor
 
 .. method:: AbstractEventLoop.set_default_executor(executor)
 
-   Set the default executor used by :meth:`run_in_executor`.
+   Set *executor* as the default executor used by :meth:`run_in_executor`.
+   *executor* should be an instance of
+   :class:`~concurrent.futures.ThreadPoolExecutor`.
+
+   .. deprecated:: 3.8
+      Using an executor that is not an instance of
+      :class:`~concurrent.futures.ThreadPoolExecutor` is deprecated and
+      will trigger an error in Python 3.9.
 
 
 Error Handling API
diff --git a/Doc/whatsnew/3.8.rst b/Doc/whatsnew/3.8.rst
index 3f51535..bab1e06 100644
--- a/Doc/whatsnew/3.8.rst
+++ b/Doc/whatsnew/3.8.rst
@@ -164,6 +164,12 @@ Deprecated
   They will be removed in Python 3.9.
   (Contributed by Serhiy Storchaka in :issue:`29209`.)
 
+* Passing an object that is not an instance of
+  :class:`concurrent.futures.ThreadPoolExecutor` to
+  :meth:`asyncio.AbstractEventLoop.set_default_executor()` is
+  deprecated and will be prohibited in Python 3.9.
+  (Contributed by Elvis Pranskevichus in :issue:`34075`.)
+
 
 Removed
 =======