bpo-45410: Add test.support.flush_std_streams() (GH-28885)

support.print_warning() now flushs sys.stdout.

Doc/library/test.rst

@@ -607,6 +607,15 @@ The :mod:`test.support` module defines the following functions:
    target of the "as" clause, if there is one.
 
 
+.. function:: flush_std_streams()
+
+   Call the ``flush()`` method on :data:`sys.stdout` and then on
+   :data:`sys.stderr`. It can be used to make sure that the logs order is
+   consistent before writing into stderr.
+
+   .. versionadded:: 3.11
+
+
 .. function:: print_warning(msg)
 
    Print a warning into :data:`sys.__stderr__`. Format the message as:

Lib/test/libregrtest/utils.py

@@ -68,20 +68,13 @@ def print_warning(msg):
 orig_unraisablehook = None
 
 
-def flush_std_streams():
-    if sys.stdout is not None:
-        sys.stdout.flush()
-    if sys.stderr is not None:
-        sys.stderr.flush()
-
-
 def regrtest_unraisable_hook(unraisable):
     global orig_unraisablehook
     support.environment_altered = True
     print_warning("Unraisable exception")
     old_stderr = sys.stderr
     try:
-        flush_std_streams()
+        support.flush_std_streams()
         sys.stderr = sys.__stderr__
         orig_unraisablehook(unraisable)
         sys.stderr.flush()
@@ -104,7 +97,7 @@ def regrtest_threading_excepthook(args):
     print_warning(f"Uncaught thread exception: {args.exc_type.__name__}")
     old_stderr = sys.stderr
     try:
-        flush_std_streams()
+        support.flush_std_streams()
         sys.stderr = sys.__stderr__
         orig_threading_excepthook(args)
         sys.stderr.flush()

Lib/test/support/__init__.py

@@ -1167,7 +1167,16 @@ def run_doctest(module, verbosity=None, optionflags=0):
 #=======================================================================
 # Support for saving and restoring the imported modules.
 
+def flush_std_streams():
+    if sys.stdout is not None:
+        sys.stdout.flush()
+    if sys.stderr is not None:
+        sys.stderr.flush()
+
+
 def print_warning(msg):
+    # bpo-45410: Explicitly flush stdout to keep logs in order
+    flush_std_streams()
     # bpo-39983: Print into sys.__stderr__ to display the warning even
     # when sys.stderr is captured temporarily by a test
     for line in msg.splitlines():