Issue #21425: Fix flushing of standard streams in the interactive interpreter.

Lib/test/test_cmd_line_script.py

@@ -1,5 +1,6 @@
 # tests command line execution of scripts
 
+import contextlib
 import importlib
 import importlib.machinery
 import zipimport
@@ -8,6 +9,7 @@ import sys
 import os
 import os.path
 import py_compile
+import subprocess
 
 import textwrap
 from test import support
@@ -173,6 +175,53 @@ class CmdLineTest(unittest.TestCase):
         expected = repr(importlib.machinery.BuiltinImporter).encode("utf-8")
         self.assertIn(expected, out)
 
+    @contextlib.contextmanager
+    def interactive_python(self, separate_stderr=False):
+        if separate_stderr:
+            p = spawn_python('-i', bufsize=1, stderr=subprocess.PIPE)
+            stderr = p.stderr
+        else:
+            p = spawn_python('-i', bufsize=1, stderr=subprocess.STDOUT)
+            stderr = p.stdout
+        try:
+            # Drain stderr until prompt
+            while True:
+                data = stderr.read(4)
+                if data == b">>> ":
+                    break
+                stderr.readline()
+            yield p
+        finally:
+            kill_python(p)
+            stderr.close()
+
+    def check_repl_stdout_flush(self, separate_stderr=False):
+        with self.interactive_python(separate_stderr) as p:
+            p.stdin.write(b"print('foo')\n")
+            p.stdin.flush()
+            self.assertEqual(b'foo', p.stdout.readline().strip())
+
+    def check_repl_stderr_flush(self, separate_stderr=False):
+        with self.interactive_python(separate_stderr) as p:
+            p.stdin.write(b"1/0\n")
+            p.stdin.flush()
+            stderr = p.stderr if separate_stderr else p.stdout
+            self.assertIn(b'Traceback ', stderr.readline())
+            self.assertIn(b'File "<stdin>"', stderr.readline())
+            self.assertIn(b'ZeroDivisionError', stderr.readline())
+
+    def test_repl_stdout_flush(self):
+        self.check_repl_stdout_flush()
+
+    def test_repl_stdout_flush_separate_stderr(self):
+        self.check_repl_stdout_flush(True)
+
+    def test_repl_stderr_flush(self):
+        self.check_repl_stderr_flush()
+
+    def test_repl_stderr_flush_separate_stderr(self):
+        self.check_repl_stderr_flush(True)
+
     def test_basic_script(self):
         with temp_dir() as script_dir:
             script_name = _make_test_script(script_dir, 'script')