Followup of #4705: we can't skip the binary buffering layer for stdin because...

Followup of #4705: we can't skip the binary buffering layer for stdin because FileIO doesn't have a read1() method

Lib/test/test_cmd_line.py

@@ -159,6 +159,16 @@ class CmdLineTest(unittest.TestCase):
             self.assertEqual(data.strip(), b'x',
                 "text %s not line-buffered" % stream)
 
+    def test_unbuffered_input(self):
+        # sys.stdin still works with '-u'
+        code = ("import sys; sys.stdout.write(sys.stdin.read(1))")
+        p = _spawn_python('-u', '-c', code)
+        p.stdin.write(b'x')
+        p.stdin.flush()
+        data, rc = _kill_python_and_exit_code(p)
+        self.assertEqual(rc, 0)
+        self.assertEqual(data.strip(), b'x')
+
 
 def test_main():
     test.support.run_unittest(CmdLineTest)

Python/pythonrun.c

@@ -739,7 +739,12 @@ create_stdio(PyObject* io,
 	PyObject *line_buffering;
 	int buffering, isatty;
 
-	if (Py_UnbufferedStdioFlag)
+	/* stdin is always opened in buffered mode, first because it shouldn't
+	   make a difference in common use cases, second because TextIOWrapper
+	   depends on the presence of a read1() method which only exists on
+	   buffered streams.
+	*/
+	if (Py_UnbufferedStdioFlag && write_mode)
 		buffering = 0;
 	else
 		buffering = -1;
@@ -753,7 +758,7 @@ create_stdio(PyObject* io,
 	if (buf == NULL)
 		goto error;
 
-	if (!Py_UnbufferedStdioFlag) {
+	if (buffering) {
 		raw = PyObject_GetAttrString(buf, "raw");
 		if (raw == NULL)
 			goto error;