bpo-13601: always use line-buffering for sys.stderr (GH-17646)

1 files changed, 9 insertions, 3 deletions

diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index a824fb9..0aae263 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -1446,9 +1446,15 @@ always available.
      for the Windows console, this only applies when
      :envvar:`PYTHONLEGACYWINDOWSSTDIO` is also set.
 
-   * When interactive, ``stdout`` and ``stderr`` streams are line-buffered.
-     Otherwise, they are block-buffered like regular text files.  You can
-     override this value with the :option:`-u` command-line option.
+   * When interactive, the ``stdout`` stream is line-buffered. Otherwise,
+     it is block-buffered like regular text files.  The ``stderr`` stream
+     is line-buffered in both cases.  You can make both streams unbuffered
+     by passing the :option:`-u` command-line option or setting the
+     :envvar:`PYTHONUNBUFFERED` environment variable.
+
+   .. versionchanged:: 3.9
+      Non-interactive ``stderr`` is now line-buffered instead of fully
+      buffered.
 
    .. note::