Use a marker instead of a string so it can be distingished when used as stdin.

subprocess2.py

 # coding=utf8
 # Copyright (c) 2011 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 """Collection of subprocess wrapper functions.
 
 In theory you shouldn't need anything else in subprocess, or this module failed.
 """
 
 from __future__ import with_statement
 import errno
 import logging
 import os
 import subprocess
 import sys
 import tempfile
 import time
 import threading
 
 # Constants forwarded from subprocess.
 PIPE = subprocess.PIPE
 STDOUT = subprocess.STDOUT
 # Sends stdout or stderr to os.devnull.
-VOID = '/dev/null'
+VOID = object()
 # Error code when a process was killed because it timed out.
 TIMED_OUT = -2001
 
 # Globals.
 # Set to True if you somehow need to disable this hack.
 SUBPROCESS_CLEANUP_HACKED = False
 
 
 class CalledProcessError(subprocess.CalledProcessError):
   """Augment the standard exception with more data."""
@@ skipping 161 matching lines @@
   """Wraps subprocess.Popen().communicate().
 
   Returns ((stdout, stderr), returncode).
 
   - The process will be killed after |timeout| seconds and returncode set to
     TIMED_OUT.
   - Automatically passes stdin content as input so do not specify stdin=PIPE.
   """
   stdin = kwargs.pop('stdin', None)
   if stdin is not None:
-    assert stdin != PIPE
-    # When stdin is passed as an argument, use it as the actual input data and
-    # set the Popen() parameter accordingly.
-    kwargs['stdin'] = PIPE
+    if stdin is VOID:
+      kwargs['stdin'] = open(os.devnull, 'r')
+      stdin = None
+    else:
+      assert isinstance(stdin, str)
+      # When stdin is passed as an argument, use it as the actual input data and
+      # set the Popen() parameter accordingly.
+      kwargs['stdin'] = PIPE
 
   if not timeout:
     # Normal workflow.
     proc = Popen(args, **kwargs)
     if stdin is not None:
       return proc.communicate(stdin), proc.returncode
     else:
       return proc.communicate(), proc.returncode
 
   # Create a temporary file to workaround python's deadlock.
@@ skipping 60 matching lines @@
   - Works even prior to python 2.7.
   - Blocks stdin by default since no output will be visible.
   """
   if kwargs.get('stdin') is None:
     kwargs['stdin'] = VOID
   if kwargs.get('stdout') is None:
     kwargs['stdout'] = PIPE
   if kwargs.get('stderr') is None:
     kwargs['stderr'] = STDOUT
   return check_call(args, **kwargs)[0]