Issue #5870: Add subprocess.DEVNULL constant.

Doc/library/subprocess.rst

@@ -125,12 +125,14 @@ This module defines one class called :class:`Popen`:
 
    *stdin*, *stdout* and *stderr* specify the executed programs' standard input,
    standard output and standard error file handles, respectively.  Valid values
-   are :data:`PIPE`, an existing file descriptor (a positive integer), an
-   existing :term:`file object`, and ``None``.  :data:`PIPE` indicates that a
-   new pipe to the child should be created.  With ``None``, no redirection will
-   occur; the child's file handles will be inherited from the parent.  Additionally,
-   *stderr* can be :data:`STDOUT`, which indicates that the stderr data from the
-   applications should be captured into the same file handle as for stdout.
+   are :data:`PIPE`, :data:`DEVNULL`, an existing file descriptor (a positive
+   integer), an existing :term:`file object`, and ``None``.  :data:`PIPE`
+   indicates that a new pipe to the child should be created.  :data:`DEVNULL`
+   indicates that the special file :data:`os.devnull` will be used. With ``None``,
+   no redirection will occur; the child's file handles will be inherited from
+   the parent.  Additionally, *stderr* can be :data:`STDOUT`, which indicates
+   that the stderr data from the applications should be captured into the same
+   file handle as for stdout.
 
    If *preexec_fn* is set to a callable object, this object will be called in the
    child process just before the child is executed.
@@ -229,6 +231,15 @@ This module defines one class called :class:`Popen`:
       Added context manager support.
 
 
+.. data:: DEVNULL
+
+   Special value that can be used as the *stdin*, *stdout* or *stderr* argument
+   to :class:`Popen` and indicates that the special file :data:`os.devnull`
+   will be used.
+
+   .. versionadded:: 3.3
+
+
 .. data:: PIPE
 
    Special value that can be used as the *stdin*, *stdout* or *stderr* argument

Lib/subprocess.py

@@ -431,7 +431,7 @@ else:
             return fds
 
 __all__ = ["Popen", "PIPE", "STDOUT", "call", "check_call", "getstatusoutput",
-           "getoutput", "check_output", "CalledProcessError"]
+           "getoutput", "check_output", "CalledProcessError", "DEVNULL"]
 
 if mswindows:
     from _subprocess import CREATE_NEW_CONSOLE, CREATE_NEW_PROCESS_GROUP
@@ -456,6 +456,7 @@ def _cleanup():
 
 PIPE = -1
 STDOUT = -2
+DEVNULL = -3
 
 
 def _eintr_retry_call(func, *args):
@@ -800,6 +801,10 @@ class Popen(object):
             # Child is still running, keep us alive until we can wait on it.
             _active.append(self)
 
+    def _get_devnull(self):
+        if not hasattr(self, '_devnull'):
+            self._devnull = os.open(os.devnull, os.O_RDWR)
+        return self._devnull
 
     def communicate(self, input=None, timeout=None):
         """Interact with process: Send data to stdin.  Read data from
@@ -889,6 +894,8 @@ class Popen(object):
                     p2cread, _ = _subprocess.CreatePipe(None, 0)
             elif stdin == PIPE:
                 p2cread, p2cwrite = _subprocess.CreatePipe(None, 0)
+            elif stdin == DEVNULL:
+                p2cread = msvcrt.get_osfhandle(self._get_devnull())
             elif isinstance(stdin, int):
                 p2cread = msvcrt.get_osfhandle(stdin)
             else:
@@ -902,6 +909,8 @@ class Popen(object):
                     _, c2pwrite = _subprocess.CreatePipe(None, 0)
             elif stdout == PIPE:
                 c2pread, c2pwrite = _subprocess.CreatePipe(None, 0)
+            elif stdout == DEVNULL:
+                c2pwrite = msvcrt.get_osfhandle(self._get_devnull())
             elif isinstance(stdout, int):
                 c2pwrite = msvcrt.get_osfhandle(stdout)
             else:
@@ -917,6 +926,8 @@ class Popen(object):
                 errread, errwrite = _subprocess.CreatePipe(None, 0)
             elif stderr == STDOUT:
                 errwrite = c2pwrite
+            elif stderr == DEVNULL:
+                errwrite = msvcrt.get_osfhandle(self._get_devnull())
             elif isinstance(stderr, int):
                 errwrite = msvcrt.get_osfhandle(stderr)
             else:
@@ -1026,6 +1037,8 @@ class Popen(object):
                     c2pwrite.Close()
                 if errwrite != -1:
                     errwrite.Close()
+                if hasattr(self, '_devnull'):
+                    os.close(self._devnull)
 
             # Retain the process handle, but close the thread handle
             self._child_created = True
@@ -1159,6 +1172,8 @@ class Popen(object):
                 pass
             elif stdin == PIPE:
                 p2cread, p2cwrite = _create_pipe()
+            elif stdin == DEVNULL:
+                p2cread = self._get_devnull()
             elif isinstance(stdin, int):
                 p2cread = stdin
             else:
@@ -1169,6 +1184,8 @@ class Popen(object):
                 pass
             elif stdout == PIPE:
                 c2pread, c2pwrite = _create_pipe()
+            elif stdout == DEVNULL:
+                c2pwrite = self._get_devnull()
             elif isinstance(stdout, int):
                 c2pwrite = stdout
             else:
@@ -1181,6 +1198,8 @@ class Popen(object):
                 errread, errwrite = _create_pipe()
             elif stderr == STDOUT:
                 errwrite = c2pwrite
+            elif stderr == DEVNULL:
+                errwrite = self._get_devnull()
             elif isinstance(stderr, int):
                 errwrite = stderr
             else:
@@ -1374,6 +1393,8 @@ class Popen(object):
                     os.close(c2pwrite)
                 if errwrite != -1 and errread != -1:
                     os.close(errwrite)
+                if hasattr(self, '_devnull'):
+                    os.close(self._devnull)
 
                 # Wait for exec to fail or succeed; possibly raising an
                 # exception (limited in size)

Lib/test/test_subprocess.py

@@ -323,6 +323,31 @@ class ProcessTestCase(BaseTestCase):
         rc = subprocess.call([sys.executable, "-c", cmd], stdout=1)
         self.assertEqual(rc, 2)
 
+    def test_stdout_devnull(self):
+        p = subprocess.Popen([sys.executable, "-c",
+                              'for i in range(10240):'
+                              'print("x" * 1024)'],
+                              stdout=subprocess.DEVNULL)
+        p.wait()
+        self.assertEqual(p.stdout, None)
+
+    def test_stderr_devnull(self):
+        p = subprocess.Popen([sys.executable, "-c",
+                              'import sys\n'
+                              'for i in range(10240):'
+                              'sys.stderr.write("x" * 1024)'],
+                              stderr=subprocess.DEVNULL)
+        p.wait()
+        self.assertEqual(p.stderr, None)
+
+    def test_stdin_devnull(self):
+        p = subprocess.Popen([sys.executable, "-c",
+                              'import sys;'
+                              'sys.stdin.read(1)'],
+                              stdin=subprocess.DEVNULL)
+        p.wait()
+        self.assertEqual(p.stdin, None)
+
     def test_cwd(self):
         tmpdir = tempfile.gettempdir()
         # We cannot use os.path.realpath to canonicalize the path,

Misc/NEWS

@@ -72,6 +72,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #5870: Add `subprocess.DEVNULL` constant.
+
 - Issue #11133: fix two cases where inspect.getattr_static can trigger code
   execution. Patch by Andreas Stührk.