return the new file descriptor from os.dup2 (closes bpo-32441)

Author: benjaminp

Doc/library/os.rst

@@ -735,13 +735,17 @@ as internal buffering of data.
 
 .. function:: dup2(fd, fd2, inheritable=True)
 
-   Duplicate file descriptor *fd* to *fd2*, closing the latter first if necessary.
-   The file descriptor *fd2* is :ref:`inheritable <fd_inheritance>` by default,
-   or non-inheritable if *inheritable* is ``False``.
+   Duplicate file descriptor *fd* to *fd2*, closing the latter first if
+   necessary. Return *fd2*. The new file descriptor is :ref:`inheritable
+   <fd_inheritance>` by default or non-inheritable if *inheritable*
+   is ``False``.
 
    .. versionchanged:: 3.4
       Add the optional *inheritable* parameter.
 
+   .. versionchanged:: 3.7
+      Return *fd2* on success. Previously, ``None`` was always returned.
+
 
 .. function:: fchmod(fd, mode)
 

Lib/test/test_os.py

@@ -3079,19 +3079,15 @@ def test_dup2(self):
 
         # inheritable by default
         fd2 = os.open(__file__, os.O_RDONLY)
-        try:
-            os.dup2(fd, fd2)
-            self.assertEqual(os.get_inheritable(fd2), True)
-        finally:
-            os.close(fd2)
+        self.addCleanup(os.close, fd2)
+        self.assertEqual(os.dup2(fd, fd2), fd2)
+        self.assertTrue(os.get_inheritable(fd2))
 
         # force non-inheritable
         fd3 = os.open(__file__, os.O_RDONLY)
-        try:
-            os.dup2(fd, fd3, inheritable=False)
-            self.assertEqual(os.get_inheritable(fd3), False)
-        finally:
-            os.close(fd3)
+        self.addCleanup(os.close, fd3)
+        self.assertEqual(os.dup2(fd, fd3, inheritable=False), fd3)
+        self.assertFalse(os.get_inheritable(fd3))
 
     @unittest.skipUnless(hasattr(os, 'openpty'), "need os.openpty()")
     def test_openpty(self):

Misc/NEWS.d/next/Library/2017-12-28-21-30-40.bpo-32441.LqlboJ.rst

@@ -0,0 +1,2 @@
+Return the new file descriptor (i.e., the second argument) from ``os.dup2``.
+Previously, ``None`` was always returned.

Modules/posixmodule.c

@@ -7804,6 +7804,7 @@ os_dup2_impl(PyObject *module, int fd, int fd2, int inheritable)
     Py_END_ALLOW_THREADS
     if (res < 0)
         return posix_error();
+    res = fd2; // msvcrt dup2 returns 0 on success.
 
     /* Character files like console cannot be make non-inheritable */
     if (!inheritable && _Py_set_inheritable(fd2, 0, NULL) < 0) {
@@ -7855,7 +7856,7 @@ os_dup2_impl(PyObject *module, int fd, int fd2, int inheritable)
 
 #endif
 
-    Py_RETURN_NONE;
+    return PyLong_FromLong(res);
 }