bpo-32309: Implement asyncio.to_thread() (GH-20143)

Implements `asyncio.to_thread`, a coroutine for asynchronously running IO-bound functions in a separate thread without blocking the event loop. See the discussion starting from [here](https://github.com/python/cpython/pull/18410#issuecomment-628930973) in GH-18410 for context.

Automerge-Triggered-By: @aeros

2 files changed, 23 insertions, 0 deletions

diff --git a/Lib/asyncio/__init__.py b/Lib/asyncio/__init__.py
index 28c2e2c..eb84bfb 100644
--- a/Lib/asyncio/__init__.py
+++ b/Lib/asyncio/__init__.py
@@ -17,6 +17,7 @@ from .queues import *
 from .streams import *
 from .subprocess import *
 from .tasks import *
+from .threads import *
 from .transports import *
 
 # Exposed for _asynciomodule.c to implement now deprecated
@@ -35,6 +36,7 @@ __all__ = (base_events.__all__ +
            streams.__all__ +
            subprocess.__all__ +
            tasks.__all__ +
+           threads.__all__ +
            transports.__all__)
 
 if sys.platform == 'win32':  # pragma: no cover
diff --git a/Lib/asyncio/threads.py b/Lib/asyncio/threads.py
new file mode 100644
index 0000000..2f40467
--- /dev/null
+++ b/Lib/asyncio/threads.py
@@ -0,0 +1,21 @@
+"""High-level support for working with threads in asyncio"""
+
+import functools
+
+from . import events
+
+
+__all__ = "to_thread",
+
+
+async def to_thread(func, /, *args, **kwargs):
+    """Asynchronously run function *func* in a separate thread.
+
+    Any *args and **kwargs supplied for this function are directly passed
+    to *func*.
+
+    Return an asyncio.Future which represents the eventual result of *func*.
+    """
+    loop = events.get_running_loop()
+    func_call = functools.partial(func, *args, **kwargs)
+    return await loop.run_in_executor(None, func_call)