gh-75279: Document that asyncio StreamWriter.drain() may not yield to the event loop (#155132)

https://github.com/python/cpython/commit/ab8c7372fdf9997ed5562a7506bd34af9e17f8c5
commit: ab8c7372fdf9997ed5562a7506bd34af9e17f8c5
branch: main
author: Kumar Aditya <[email protected]>
committer: kumaraditya303 <[email protected]>
date: 2026-08-03T18:07:01Z
summary:

gh-75279: Document that asyncio StreamWriter.drain() may not yield to the event loop (#155132)

files:
M Doc/library/asyncio-stream.rst

diff --git a/Doc/library/asyncio-stream.rst b/Doc/library/asyncio-stream.rst
index 4092f440f66ad3..885d463c0fca89 100644
--- a/Doc/library/asyncio-stream.rst
+++ b/Doc/library/asyncio-stream.rst
@@ -389,6 +389,16 @@ StreamWriter
       be resumed.  When there is nothing to wait for, the :meth:`drain`
       returns immediately.
 
+      .. note::
+
+         When the write buffer is below the high watermark,
+         :meth:`drain` returns immediately without yielding to
+         the event loop.  As a result, code which repeatedly calls
+         ``write()`` followed by ``await drain()`` may prevent other
+         tasks from running.  To prevent blocking behavior, yield
+         to the event loop explicitly with ``await asyncio.sleep(0)``
+         (see :func:`asyncio.sleep`).
+
    .. method:: start_tls(sslcontext, *, server_hostname=None, \
                          ssl_handshake_timeout=None, ssl_shutdown_timeout=None)
       :async:

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