Issue #7111: Python can now be run without a stdin, stdout or stderr stream.

It was already the case with Python 2.  However, the corresponding
sys module entries are now set to None (instead of an unusable file object).

Lib/test/test_cmd_line.py

@@ -285,6 +285,45 @@ class CmdLineTest(unittest.TestCase):
         rc, out, err = assert_python_ok('-c', code)
         self.assertEqual(b'', err)
 
+    # Issue #7111: Python should work without standard streams
+
+    @unittest.skipIf(os.name != 'posix', "test needs POSIX semantics")
+    def _test_no_stdio(self, streams):
+        code = """if 1:
+            import os, sys
+            for i, s in enumerate({streams}):
+                if getattr(sys, s) is not None:
+                    os._exit(i + 1)
+            os._exit(42)""".format(streams=streams)
+        def preexec():
+            if 'stdin' in streams:
+                os.close(0)
+            if 'stdout' in streams:
+                os.close(1)
+            if 'stderr' in streams:
+                os.close(2)
+        p = subprocess.Popen(
+            [sys.executable, "-E", "-c", code],
+            stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            preexec_fn=preexec)
+        out, err = p.communicate()
+        self.assertEqual(test.support.strip_python_stderr(err), b'')
+        self.assertEqual(p.returncode, 42)
+
+    def test_no_stdin(self):
+        self._test_no_stdio(['stdin'])
+
+    def test_no_stdout(self):
+        self._test_no_stdio(['stdout'])
+
+    def test_no_stderr(self):
+        self._test_no_stdio(['stderr'])
+
+    def test_no_std_streams(self):
+        self._test_no_stdio(['stdin', 'stdout', 'stderr'])
+
 
 def test_main():
     test.support.run_unittest(CmdLineTest)

Misc/NEWS

@@ -10,6 +10,10 @@ What's New in Python 3.3 Alpha 1?
 Core and Builtins
 -----------------
 
+- Issue #7111: Python can now be run without a stdin, stdout or stderr
+  stream.  It was already the case with Python 2.  However, the corresponding
+  sys module entries are now set to None (instead of an unusable file object).
+
 - Issue #11849: Ensure that free()d memory arenas are really released
   on POSIX systems supporting anonymous memory mappings.  Patch by
   Charles-François Natali.

Python/pythonrun.c

@@ -912,6 +912,19 @@ error:
     return NULL;
 }
 
+static int
+is_valid_fd(int fd)
+{
+    int dummy_fd;
+    if (fd < 0 || !_PyVerify_fd(fd))
+        return 0;
+    dummy_fd = dup(fd);
+    if (dummy_fd < 0)
+        return 0;
+    close(dummy_fd);
+    return 1;
+}
+
 /* Initialize sys.stdin, stdout, stderr and builtins.open */
 static int
 initstdio(void)
@@ -971,13 +984,9 @@ initstdio(void)
      * and fileno() may point to an invalid file descriptor. For example
      * GUI apps don't have valid standard streams by default.
      */
-    if (fd < 0) {
-#ifdef MS_WINDOWS
+    if (!is_valid_fd(fd)) {
         std = Py_None;
         Py_INCREF(std);
-#else
-        goto error;
-#endif
     }
     else {
         std = create_stdio(iomod, fd, 0, "<stdin>", encoding, errors);
@@ -990,13 +999,9 @@ initstdio(void)
 
     /* Set sys.stdout */
     fd = fileno(stdout);
-    if (fd < 0) {
-#ifdef MS_WINDOWS
+    if (!is_valid_fd(fd)) {
         std = Py_None;
         Py_INCREF(std);
-#else
-        goto error;
-#endif
     }
     else {
         std = create_stdio(iomod, fd, 1, "<stdout>", encoding, errors);
@@ -1010,13 +1015,9 @@ initstdio(void)
 #if 1 /* Disable this if you have trouble debugging bootstrap stuff */
     /* Set sys.stderr, replaces the preliminary stderr */
     fd = fileno(stderr);
-    if (fd < 0) {
-#ifdef MS_WINDOWS
+    if (!is_valid_fd(fd)) {
         std = Py_None;
         Py_INCREF(std);
-#else
-        goto error;
-#endif
     }
     else {
         std = create_stdio(iomod, fd, 1, "<stderr>", encoding, "backslashreplace");