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

1 files changed, 16 insertions, 0 deletions

diff --git a/Lib/test/test_cmd_line.py b/Lib/test/test_cmd_line.py
index 4781002..ee96473 100644
--- a/Lib/test/test_cmd_line.py
+++ b/Lib/test/test_cmd_line.py
@@ -6,6 +6,7 @@ import os
 import subprocess
 import sys
 import tempfile
+import textwrap
 import unittest
 from test import support
 from test.support.script_helper import (
@@ -219,6 +220,21 @@ class CmdLineTest(unittest.TestCase):
         )
         check_output(text)
 
+    def test_non_interactive_output_buffering(self):
+        code = textwrap.dedent("""
+            import sys
+            out = sys.stdout
+            print(out.isatty(), out.write_through, out.line_buffering)
+            err = sys.stderr
+            print(err.isatty(), err.write_through, err.line_buffering)
+        """)
+        args = [sys.executable, '-c', code]
+        proc = subprocess.run(args, stdout=subprocess.PIPE,
+                              stderr=subprocess.PIPE, text=True, check=True)
+        self.assertEqual(proc.stdout,
+                         'False False False\n'
+                         'False False True\n')
+
     def test_unbuffered_output(self):
         # Test expected operation of the '-u' switch
         for stream in ('stdout', 'stderr'):