python/lib/Lib/subprocess.py

# subprocess - Subprocesses with accessible I/O streams
#
# For more information about this module, see PEP 324.
#
# This module should remain compatible with Python 2.2, see PEP 291.
#
# Copyright (c) 2003-2005 by Peter Astrand <astrand@lysator.liu.se>
#
# Licensed to PSF under a Contributor Agreement.
# See http://www.python.org/2.4/license for licensing details.

r"""subprocess - Subprocesses with accessible I/O streams

This module allows you to spawn processes, connect to their
input/output/error pipes, and obtain their return codes.  This module
intends to replace several other, older modules and functions, like:

os.system
os.spawn*
os.popen*
popen2.*
commands.*

Information about how the subprocess module can be used to replace these
modules and functions can be found below.



Using the subprocess module
===========================
This module defines one class called Popen:

class Popen(args, bufsize=0, executable=None,
            stdin=None, stdout=None, stderr=None,
            preexec_fn=None, close_fds=False, shell=False,
            cwd=None, env=None, universal_newlines=False,
            startupinfo=None, creationflags=0):


Arguments are:

args should be a string, or a sequence of program arguments.  The
program to execute is normally the first item in the args sequence or
string, but can be explicitly set by using the executable argument.

On UNIX, with shell=False (default): In this case, the Popen class
uses os.execvp() to execute the child program.  args should normally
be a sequence.  A string will be treated as a sequence with the string
as the only item (the program to execute).

On UNIX, with shell=True: If args is a string, it specifies the
command string to execute through the shell.  If args is a sequence,
the first item specifies the command string, and any additional items
will be treated as additional shell arguments.

On Windows: the Popen class uses CreateProcess() to execute the child
program, which operates on strings.  If args is a sequence, it will be
converted to a string using the list2cmdline method.  Please note that
not all MS Windows applications interpret the command line the same
way: The list2cmdline is designed for applications using the same
rules as the MS C runtime.

bufsize, if given, has the same meaning as the corresponding argument
to the built-in open() function: 0 means unbuffered, 1 means line
buffered, any other positive value means use a buffer of
(approximately) that size.  A negative bufsize means to use the system
default, which usually means fully buffered.  The default value for
bufsize is 0 (unbuffered).

stdin, stdout and stderr specify the executed programs' standard
input, standard output and standard error file handles, respectively.
Valid values are PIPE, an existing file descriptor (a positive
integer), an existing file object, and None.  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 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.

If close_fds is true, all file descriptors except 0, 1 and 2 will be
closed before the child process is executed.

if shell is true, the specified command will be executed through the
shell.

If cwd is not None, the current directory will be changed to cwd
before the child is executed.

If env is not None, it defines the environment variables for the new
process.

If universal_newlines is true, the file objects stdout and stderr are
opened as a text files, but lines may be terminated by any of '\n',
the Unix end-of-line convention, '\r', the Macintosh convention or
'\r\n', the Windows convention.  All of these external representations
are seen as '\n' by the Python program.  Note: This feature is only
available if Python is built with universal newline support (the
default).  Also, the newlines attribute of the file objects stdout,
stdin and stderr are not updated by the communicate() method.

The startupinfo and creationflags, if given, will be passed to the
underlying CreateProcess() function.  They can specify things such as
appearance of the main window and priority for the new process.
(Windows only)


This module also defines two shortcut functions:

call(*popenargs, **kwargs):
    Run command with arguments.  Wait for command to complete, then
    return the returncode attribute.

    The arguments are the same as for the Popen constructor.  Example:

    retcode = call(["ls", "-l"])

check_call(*popenargs, **kwargs):
    Run command with arguments.  Wait for command to complete.  If the
    exit code was zero then return, otherwise raise
    CalledProcessError.  The CalledProcessError object will have the
    return code in the returncode attribute.

    The arguments are the same as for the Popen constructor.  Example:

    check_call(["ls", "-l"])

Exceptions
----------
Exceptions raised in the child process, before the new program has
started to execute, will be re-raised in the parent.  Additionally,
the exception object will have one extra attribute called
'child_traceback', which is a string containing traceback information
from the childs point of view.

The most common exception raised is OSError.  This occurs, for
example, when trying to execute a non-existent file.  Applications
should prepare for OSErrors.

A ValueError will be raised if Popen is called with invalid arguments.

check_call() will raise CalledProcessError, if the called process
returns a non-zero return code.


Security
--------
Unlike some other popen functions, this implementation will never call
/bin/sh implicitly.  This means that all characters, including shell
metacharacters, can safely be passed to child processes.


Popen objects
=============
Instances of the Popen class have the following methods:

poll()
    Check if child process has terminated.  Returns returncode
    attribute.

wait()
    Wait for child process to terminate.  Returns returncode attribute.

communicate(input=None)
    Interact with process: Send data to stdin.  Read data from stdout
    and stderr, until end-of-file is reached.  Wait for process to
    terminate.  The optional input argument should be a string to be
    sent to the child process, or None, if no data should be sent to
    the child.

    communicate() returns a tuple (stdout, stderr).

    Note: The data read is buffered in memory, so do not use this
    method if the data size is large or unlimited.

The following attributes are also available:

stdin
    If the stdin argument is PIPE, this attribute is a file object
    that provides input to the child process.  Otherwise, it is None.

stdout
    If the stdout argument is PIPE, this attribute is a file object
    that provides output from the child process.  Otherwise, it is
    None.

stderr
    If the stderr argument is PIPE, this attribute is file object that
    provides error output from the child process.  Otherwise, it is
    None.

pid
    The process ID of the child process.

returncode
    The child return code.  A None value indicates that the process
    hasn't terminated yet.  A negative value -N indicates that the
    child was terminated by signal N (UNIX only).


Replacing older functions with the subprocess module
====================================================
In this section, "a ==> b" means that b can be used as a replacement
for a.

Note: All functions in this section fail (more or less) silently if
the executed program cannot be found; this module raises an OSError
exception.

In the following examples, we assume that the subprocess module is
imported with "from subprocess import *".


Replacing /bin/sh shell backquote
---------------------------------
output=`mycmd myarg`
==>
output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]


Replacing shell pipe line
-------------------------
output=`dmesg | grep hda`
==>
p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
output = p2.communicate()[0]


Replacing os.system()
---------------------
sts = os.system("mycmd" + " myarg")
==>
p = Popen("mycmd" + " myarg", shell=True)
pid, sts = os.waitpid(p.pid, 0)

Note:

* Calling the program through the shell is usually not required.

* It's easier to look at the returncode attribute than the
  exitstatus.

A more real-world example would look like this:

try:
    retcode = call("mycmd" + " myarg", shell=True)
    if retcode < 0:
        print >>sys.stderr, "Child was terminated by signal", -retcode
    else:
        print >>sys.stderr, "Child returned", retcode
except OSError, e:
    print >>sys.stderr, "Execution failed:", e


Replacing os.spawn*
-------------------
P_NOWAIT example:

pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
==>
pid = Popen(["/bin/mycmd", "myarg"]).pid


P_WAIT example:

retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
==>
retcode = call(["/bin/mycmd", "myarg"])


Vector example:

os.spawnvp(os.P_NOWAIT, path, args)
==>
Popen([path] + args[1:])


Environment example:

os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
==>
Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})


Replacing os.popen*
-------------------
pipe = os.popen(cmd, mode='r', bufsize)
==>
pipe = Popen(cmd, shell=True, bufsize=bufsize, stdout=PIPE).stdout

pipe = os.popen(cmd, mode='w', bufsize)
==>
pipe = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE).stdin


(child_stdin, child_stdout) = os.popen2(cmd, mode, bufsize)
==>
p = Popen(cmd, shell=True, bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdin, child_stdout) = (p.stdin, p.stdout)


(child_stdin,
 child_stdout,
 child_stderr) = os.popen3(cmd, mode, bufsize)
==>
p = Popen(cmd, shell=True, bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)
(child_stdin,
 child_stdout,
 child_stderr) = (p.stdin, p.stdout, p.stderr)


(child_stdin, child_stdout_and_stderr) = os.popen4(cmd, mode, bufsize)
==>
p = Popen(cmd, shell=True, bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
(child_stdin, child_stdout_and_stderr) = (p.stdin, p.stdout)


Replacing popen2.*
------------------
Note: If the cmd argument to popen2 functions is a string, the command
is executed through /bin/sh.  If it is a list, the command is directly
executed.

(child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode)
==>
p = Popen(["somestring"], shell=True, bufsize=bufsize
          stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdout, child_stdin) = (p.stdout, p.stdin)


(child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize, mode)
==>
p = Popen(["mycmd", "myarg"], bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdout, child_stdin) = (p.stdout, p.stdin)

The popen2.Popen3 and popen2.Popen4 basically works as subprocess.Popen,
except that:

* subprocess.Popen raises an exception if the execution fails
* the capturestderr argument is replaced with the stderr argument.
* stdin=PIPE and stdout=PIPE must be specified.
* popen2 closes all filedescriptors by default, but you have to specify
  close_fds=True with subprocess.Popen.
"""

import sys
mswindows = (sys.platform == "win32")
jython = sys.platform.startswith("java")

import os
import types
import traceback

# Exception classes used by this module.
class CalledProcessError(Exception):
    """This exception is raised when a process run by check_call() returns
    a non-zero exit status.  The exit status will be stored in the
    returncode attribute."""
    def __init__(self, returncode, cmd):
        self.returncode = returncode
        self.cmd = cmd
    def __str__(self):
        return "Command '%s' returned non-zero exit status %d" % (self.cmd, self.returncode)


if mswindows:
    import threading
    import msvcrt
    if 0: # <-- change this to use pywin32 instead of the _subprocess driver
        import pywintypes
        from win32api import GetStdHandle, STD_INPUT_HANDLE, \
                             STD_OUTPUT_HANDLE, STD_ERROR_HANDLE
        from win32api import GetCurrentProcess, DuplicateHandle, \
                             GetModuleFileName, GetVersion
        from win32con import DUPLICATE_SAME_ACCESS, SW_HIDE
        from win32pipe import CreatePipe
        from win32process import CreateProcess, STARTUPINFO, \
                                 GetExitCodeProcess, STARTF_USESTDHANDLES, \
                                 STARTF_USESHOWWINDOW, CREATE_NEW_CONSOLE
        from win32event import WaitForSingleObject, INFINITE, WAIT_OBJECT_0
    else:
        from _subprocess import *
        class STARTUPINFO:
            dwFlags = 0
            hStdInput = None
            hStdOutput = None
            hStdError = None
            wShowWindow = 0
        class pywintypes:
            error = IOError
elif jython:
    import errno
    import threading
    import java.io.File
    import java.io.IOException
    import java.lang.IllegalArgumentException
    import java.lang.IllegalThreadStateException
    import java.lang.ProcessBuilder
    import java.lang.System
    import java.lang.Thread
    import java.nio.ByteBuffer
    import org.python.core.io.RawIOBase
    import org.python.core.io.StreamIO
else:
    import select
    import errno
    import fcntl
    import gc
    import pickle

__all__ = ["Popen", "PIPE", "STDOUT", "call", "check_call", "CalledProcessError"]

try:
    MAXFD = os.sysconf("SC_OPEN_MAX")
except:
    MAXFD = 256

# True/False does not exist on 2.2.0
try:
    False
except NameError:
    False = 0
    True = 1

_active = []

def _cleanup():
    for inst in _active[:]:
        if inst.poll(_deadstate=sys.maxint) >= 0:
            try:
                _active.remove(inst)
            except ValueError:
                # This can happen if two threads create a new Popen instance.
                # It's harmless that it was already removed, so ignore.
                pass

PIPE = -1
STDOUT = -2


def call(*popenargs, **kwargs):
    """Run command with arguments.  Wait for command to complete, then
    return the returncode attribute.

    The arguments are the same as for the Popen constructor.  Example:

    retcode = call(["ls", "-l"])
    """
    return Popen(*popenargs, **kwargs).wait()


def check_call(*popenargs, **kwargs):
    """Run command with arguments.  Wait for command to complete.  If
    the exit code was zero then return, otherwise raise
    CalledProcessError.  The CalledProcessError object will have the
    return code in the returncode attribute.

    The arguments are the same as for the Popen constructor.  Example:

    check_call(["ls", "-l"])
    """
    retcode = call(*popenargs, **kwargs)
    cmd = kwargs.get("args")
    if cmd is None:
        cmd = popenargs[0]
    if retcode:
        raise CalledProcessError(retcode, cmd)
    return retcode


def list2cmdline(seq):
    """
    Translate a sequence of arguments into a command line
    string, using the same rules as the MS C runtime:

    1) Arguments are delimited by white space, which is either a
       space or a tab.

    2) A string surrounded by double quotation marks is
       interpreted as a single argument, regardless of white space
       or pipe characters contained within.  A quoted string can be
       embedded in an argument.

    3) A double quotation mark preceded by a backslash is
       interpreted as a literal double quotation mark.

    4) Backslashes are interpreted literally, unless they
       immediately precede a double quotation mark.

    5) If backslashes immediately precede a double quotation mark,
       every pair of backslashes is interpreted as a literal
       backslash.  If the number of backslashes is odd, the last
       backslash escapes the next double quotation mark as
       described in rule 3.
    """

    # See
    # http://msdn.microsoft.com/library/en-us/vccelng/htm/progs_12.asp
    result = []
    needquote = False
    for arg in seq:
        bs_buf = []

        # Add a space to separate this argument from the others
        if result:
            result.append(' ')

        needquote = (" " in arg) or ("\t" in arg) or ("|" in arg) or not arg
        if needquote:
            result.append('"')

        for c in arg:
            if c == '\\':
                # Don't know if we need to double yet.
                bs_buf.append(c)
            elif c == '"':
                # Double backslashes.
                result.append('\\' * len(bs_buf)*2)
                bs_buf = []
                result.append('\\"')
            else:
                # Normal char
                if bs_buf:
                    result.extend(bs_buf)
                    bs_buf = []
                result.append(c)

        # Add remaining backslashes, if any.
        if bs_buf:
            result.extend(bs_buf)

        if needquote:
            result.extend(bs_buf)
            result.append('"')

    return ''.join(result)


if jython:
    # Parse command line arguments for Windows
    _win_oses = ['nt']

    _cmdline2listimpl = None
    _escape_args = None
    _shell_command = None

    def _cmdline2list(cmdline):
        """Build an argv list from a Microsoft shell style cmdline str

        The reverse of list2cmdline that follows the same MS C runtime
        rules.

        Java's ProcessBuilder takes a List<String> cmdline that's joined
        with a list2cmdline-like routine for Windows CreateProcess
        (which takes a String cmdline). This process ruins String
        cmdlines from the user with escapes or quotes. To avoid this we
        first parse these cmdlines into an argv.

        Runtime.exec(String) is too naive and useless for this case.
        """
        whitespace = ' \t'
        # count of preceding '\'
        bs_count = 0
        in_quotes = False
        arg = []
        argv = []

        for ch in cmdline:
            if ch in whitespace and not in_quotes:
                if arg:
                    # finalize arg and reset
                    argv.append(''.join(arg))
                    arg = []
                bs_count = 0
            elif ch == '\\':
                arg.append(ch)
                bs_count += 1
            elif ch == '"':
                if not bs_count % 2:
                    # Even number of '\' followed by a '"'. Place one
                    # '\' for every pair and treat '"' as a delimiter
                    if bs_count:
                        del arg[-(bs_count / 2):]
                    in_quotes = not in_quotes
                else:
                    # Odd number of '\' followed by a '"'. Place one '\'
                    # for every pair and treat '"' as an escape sequence
                    # by the remaining '\'
                    del arg[-(bs_count / 2 + 1):]
                    arg.append(ch)
                bs_count = 0
            else:
                # regular char
                arg.append(ch)
                bs_count = 0

        # A single trailing '"' delimiter yields an empty arg
        if arg or in_quotes:
            argv.append(''.join(arg))

        return argv

    def _setup_platform():
        """Setup the shell command and the command line argument escape
        function depending on the underlying platform
        """
        global _cmdline2listimpl, _escape_args, _shell_command

        if os._name in _win_oses:
            _cmdline2listimpl = _cmdline2list
            _escape_args = lambda args: [list2cmdline([arg]) for arg in args]
        else:
            _cmdline2listimpl = lambda args: [args]
            _escape_args = lambda args: args

        for shell_command in os._get_shell_commands():
            executable = shell_command[0]
            if not os.path.isabs(executable):
                import distutils.spawn
                executable = distutils.spawn.find_executable(executable)
            if not executable or not os.path.exists(executable):
                continue
            shell_command[0] = executable
            _shell_command = shell_command
            return

        if not _shell_command:
            import warnings
            warnings.warn('Unable to determine _shell_command for '
                          'underlying os: %s' % os._name, RuntimeWarning, 3)
    _setup_platform()


    class _CouplerThread(java.lang.Thread):

        """Couples a reader and writer RawIOBase.

        Streams data from the reader's read_func (a RawIOBase readinto
        method) to the writer's write_func (a RawIOBase write method) in
        a separate thread. Optionally calls close_func when finished
        streaming or an exception occurs.

        This thread will fail safe when interrupted by Java's
        Thread.interrupt.
        """

        # analagous to PC_PIPE_BUF, which is typically 512 or 4096
        bufsize = 4096

        def __init__(self, name, read_func, write_func, close_func=None):
            self.read_func = read_func
            self.write_func = write_func
            self.close_func = close_func
            self.setName('%s-%s (%s)' % (self.__class__.__name__, id(self),
                                         name))
            self.setDaemon(True)

        def run(self):
            buf = java.nio.ByteBuffer.allocate(self.bufsize)
            while True:
                try:
                    count = self.read_func(buf)
                    if count < 1:
                        if self.close_func:
                            self.close_func()
                        break
                    buf.flip()
                    self.write_func(buf)
                    buf.flip()
                except IOError, ioe:
                    if self.close_func:
                        try:
                            self.close_func()
                        except:
                            pass
                    # XXX: hack, should really be a
                    # ClosedByInterruptError(IOError) exception
                    if str(ioe) == \
                            'java.nio.channels.ClosedByInterruptException':
                        return
                    raise


class Popen(object):
    def __init__(self, args, bufsize=0, executable=None,
                 stdin=None, stdout=None, stderr=None,
                 preexec_fn=None, close_fds=False, shell=False,
                 cwd=None, env=None, universal_newlines=False,
                 startupinfo=None, creationflags=0):
        """Create new Popen instance."""
        _cleanup()

        self._child_created = False
        if not isinstance(bufsize, (int, long)):
            raise TypeError("bufsize must be an integer")

        if mswindows:
            if preexec_fn is not None:
                raise ValueError("preexec_fn is not supported on Windows "
                                 "platforms")
            if close_fds and (stdin is not None or stdout is not None or
                              stderr is not None):
                raise ValueError("close_fds is not supported on Windows "
                                 "platforms if you redirect stdin/stdout/stderr")
        else:
            # POSIX
            if startupinfo is not None:
                raise ValueError("startupinfo is only supported on Windows "
                                 "platforms")
            if creationflags != 0:
                raise ValueError("creationflags is only supported on Windows "
                                 "platforms")
        if jython:
            if preexec_fn is not None:
                raise ValueError("preexec_fn is not supported on the Jython "
                                 "platform")

        self.stdin = None
        self.stdout = None
        self.stderr = None
        self.pid = None
        self.returncode = None
        self.universal_newlines = universal_newlines

        # Input and output objects. The general principle is like
        # this:
        #
        # Parent                   Child
        # ------                   -----
        # p2cwrite   ---stdin--->  p2cread
        # c2pread    <--stdout---  c2pwrite
        # errread    <--stderr---  errwrite
        #
        # On POSIX, the child objects are file descriptors.  On
        # Windows, these are Windows file handles.  The parent objects
        # are file descriptors on both platforms.  The parent objects
        # are None when not using PIPEs. The child objects are None
        # when not redirecting.

        (p2cread, p2cwrite,
         c2pread, c2pwrite,
         errread, errwrite) = self._get_handles(stdin, stdout, stderr)

        self._execute_child(args, executable, preexec_fn, close_fds,
                            cwd, env, universal_newlines,
                            startupinfo, creationflags, shell,
                            p2cread, p2cwrite,
                            c2pread, c2pwrite,
                            errread, errwrite)

        # On Windows, you cannot just redirect one or two handles: You
        # either have to redirect all three or none. If the subprocess
        # user has only redirected one or two handles, we are
        # automatically creating PIPEs for the rest. We should close
        # these after the process is started. See bug #1124861.
        if mswindows:
            if stdin is None and p2cwrite is not None:
                os.close(p2cwrite)
                p2cwrite = None
            if stdout is None and c2pread is not None:
                os.close(c2pread)
                c2pread = None
            if stderr is None and errread is not None:
                os.close(errread)
                errread = None

        if jython:
            self._stdin_thread = None
            self._stdout_thread = None
            self._stderr_thread = None

            # 'ct' is for _CouplerThread
            proc = self._process
            ct2cwrite = org.python.core.io.StreamIO(proc.getOutputStream(),
                                                    True)
            c2ctread = org.python.core.io.StreamIO(proc.getInputStream(), True)
            cterrread = org.python.core.io.StreamIO(proc.getErrorStream(),
                                                    True)

            # Use the java.lang.Process streams for PIPE, otherwise
            # direct the desired file to/from the java.lang.Process
            # streams in a separate thread
            if p2cwrite == PIPE:
                p2cwrite = ct2cwrite
            else:
                if p2cread is None:
                    # Coupling stdin is not supported: there's no way to
                    # cleanly interrupt it if it blocks the
                    # _CouplerThread forever (we can Thread.interrupt()
                    # its _CouplerThread but that closes stdin's
                    # Channel)
                    pass
                else:
                    self._stdin_thread = self._coupler_thread('stdin',
                                                              p2cread.readinto,
                                                              ct2cwrite.write,
                                                              ct2cwrite.close)
                    self._stdin_thread.start()

            if c2pread == PIPE:
                c2pread = c2ctread
            else:
                if c2pwrite is None:
                    c2pwrite = org.python.core.io.StreamIO(
                        java.lang.System.out, False)
                self._stdout_thread = self._coupler_thread('stdout',
                                                           c2ctread.readinto,
                                                           c2pwrite.write)
                self._stdout_thread.start()

            if errread == PIPE:
                errread = cterrread
            elif not self._stderr_is_stdout(errwrite, c2pwrite):
                if errwrite is None:
                    errwrite = org.python.core.io.StreamIO(
                        java.lang.System.err, False)
                self._stderr_thread = self._coupler_thread('stderr',
                                                           cterrread.readinto,
                                                           errwrite.write)
                self._stderr_thread.start()

        if p2cwrite is not None:
            self.stdin = os.fdopen(p2cwrite, 'wb', bufsize)
        if c2pread is not None:
            if universal_newlines:
                self.stdout = os.fdopen(c2pread, 'rU', bufsize)
            else:
                self.stdout = os.fdopen(c2pread, 'rb', bufsize)
        if errread is not None:
            if universal_newlines:
                self.stderr = os.fdopen(errread, 'rU', bufsize)
            else:
                self.stderr = os.fdopen(errread, 'rb', bufsize)


    def _translate_newlines(self, data):
        data = data.replace("\r\n", "\n")
        data = data.replace("\r", "\n")
        return data


    def __del__(self, sys=sys):
        if not self._child_created:
            # We didn't get to successfully create a child process.
            return
        # In case the child hasn't been waited on, check if it's done.
        self.poll(_deadstate=sys.maxint)
        if self.returncode is None and _active is not None:
            # Child is still running, keep us alive until we can wait on it.
            _active.append(self)


    def communicate(self, input=None):
        """Interact with process: Send data to stdin.  Read data from
        stdout and stderr, until end-of-file is reached.  Wait for
        process to terminate.  The optional input argument should be a
        string to be sent to the child process, or None, if no data
        should be sent to the child.

        communicate() returns a tuple (stdout, stderr)."""

        # Optimization: If we are only using one pipe, or no pipe at
        # all, using select() or threads is unnecessary.
        if [self.stdin, self.stdout, self.stderr].count(None) >= 2:
            stdout = None
            stderr = None
            if self.stdin:
                if input:
                    self.stdin.write(input)
                self.stdin.close()
            elif self.stdout:
                stdout = self.stdout.read()
            elif self.stderr:
                stderr = self.stderr.read()
            self.wait()
            return (stdout, stderr)

        return self._communicate(input)


    if mswindows or jython:
        #
        # Windows and Jython shared methods
        #
        def _readerthread(self, fh, buffer):
            buffer.append(fh.read())


        def _communicate(self, input):
            stdout = None # Return
            stderr = None # Return

            if self.stdout:
                stdout = []
                stdout_thread = threading.Thread(target=self._readerthread,
                                                 args=(self.stdout, stdout))
                stdout_thread.setDaemon(True)
                stdout_thread.start()
            if self.stderr:
                stderr = []
                stderr_thread = threading.Thread(target=self._readerthread,
                                                 args=(self.stderr, stderr))
                stderr_thread.setDaemon(True)
                stderr_thread.start()

            if self.stdin:
                if input is not None:
                    self.stdin.write(input)
                self.stdin.close()

            if self.stdout:
                stdout_thread.join()
            if self.stderr:
                stderr_thread.join()

            # All data exchanged.  Translate lists into strings.
            if stdout is not None:
                stdout = stdout[0]
            if stderr is not None:
                stderr = stderr[0]

            # Translate newlines, if requested.  We cannot let the file
            # object do the translation: It is based on stdio, which is
            # impossible to combine with select (unless forcing no
            # buffering).
            if self.universal_newlines and hasattr(file, 'newlines'):
                if stdout:
                    stdout = self._translate_newlines(stdout)
                if stderr:
                    stderr = self._translate_newlines(stderr)

            self.wait()
            return (stdout, stderr)


    if mswindows:
        #
        # Windows methods
        #
        def _get_handles(self, stdin, stdout, stderr):
            """Construct and return tupel with IO objects:
            p2cread, p2cwrite, c2pread, c2pwrite, errread, errwrite
            """
            if stdin is None and stdout is None and stderr is None:
                return (None, None, None, None, None, None)

            p2cread, p2cwrite = None, None
            c2pread, c2pwrite = None, None
            errread, errwrite = None, None

            if stdin is None:
                p2cread = GetStdHandle(STD_INPUT_HANDLE)
            if p2cread is not None:
                pass
            elif stdin is None or stdin == PIPE:
                p2cread, p2cwrite = CreatePipe(None, 0)
                # Detach and turn into fd
                p2cwrite = p2cwrite.Detach()
                p2cwrite = msvcrt.open_osfhandle(p2cwrite, 0)
            elif isinstance(stdin, int):
                p2cread = msvcrt.get_osfhandle(stdin)
            else:
                # Assuming file-like object
                p2cread = msvcrt.get_osfhandle(stdin.fileno())
            p2cread = self._make_inheritable(p2cread)

            if stdout is None:
                c2pwrite = GetStdHandle(STD_OUTPUT_HANDLE)
            if c2pwrite is not None:
                pass
            elif stdout is None or stdout == PIPE:
                c2pread, c2pwrite = CreatePipe(None, 0)
                # Detach and turn into fd
                c2pread = c2pread.Detach()
                c2pread = msvcrt.open_osfhandle(c2pread, 0)
            elif isinstance(stdout, int):
                c2pwrite = msvcrt.get_osfhandle(stdout)
            else:
                # Assuming file-like object
                c2pwrite = msvcrt.get_osfhandle(stdout.fileno())
            c2pwrite = self._make_inheritable(c2pwrite)

            if stderr is None:
                errwrite = GetStdHandle(STD_ERROR_HANDLE)
            if errwrite is not None:
                pass
            elif stderr is None or stderr == PIPE:
                errread, errwrite = CreatePipe(None, 0)
                # Detach and turn into fd
                errread = errread.Detach()
                errread = msvcrt.open_osfhandle(errread, 0)
            elif stderr == STDOUT:
                errwrite = c2pwrite
            elif isinstance(stderr, int):
                errwrite = msvcrt.get_osfhandle(stderr)
            else:
                # Assuming file-like object
                errwrite = msvcrt.get_osfhandle(stderr.fileno())
            errwrite = self._make_inheritable(errwrite)

            return (p2cread, p2cwrite,
                    c2pread, c2pwrite,
                    errread, errwrite)


        def _make_inheritable(self, handle):
            """Return a duplicate of handle, which is inheritable"""
            return DuplicateHandle(GetCurrentProcess(), handle,
                                   GetCurrentProcess(), 0, 1,
                                   DUPLICATE_SAME_ACCESS)


        def _find_w9xpopen(self):
            """Find and return absolut path to w9xpopen.exe"""
            w9xpopen = os.path.join(os.path.dirname(GetModuleFileName(0)),
                                    "w9xpopen.exe")
            if not os.path.exists(w9xpopen):
                # Eeek - file-not-found - possibly an embedding
                # situation - see if we can locate it in sys.exec_prefix
                w9xpopen = os.path.join(os.path.dirname(sys.exec_prefix),
                                        "w9xpopen.exe")
                if not os.path.exists(w9xpopen):
                    raise RuntimeError("Cannot locate w9xpopen.exe, which is "
                                       "needed for Popen to work with your "
                                       "shell or platform.")
            return w9xpopen


        def _execute_child(self, args, executable, preexec_fn, close_fds,
                           cwd, env, universal_newlines,
                           startupinfo, creationflags, shell,
                           p2cread, p2cwrite,
                           c2pread, c2pwrite,
                           errread, errwrite):
            """Execute program (MS Windows version)"""

            if not isinstance(args, types.StringTypes):
                args = list2cmdline(args)

            # Process startup details
            if startupinfo is None:
                startupinfo = STARTUPINFO()
            if None not in (p2cread, c2pwrite, errwrite):
                startupinfo.dwFlags |= STARTF_USESTDHANDLES
                startupinfo.hStdInput = p2cread
                startupinfo.hStdOutput = c2pwrite
                startupinfo.hStdError = errwrite

            if shell:
                startupinfo.dwFlags |= STARTF_USESHOWWINDOW
                startupinfo.wShowWindow = SW_HIDE
                comspec = os.environ.get("COMSPEC", "cmd.exe")
                args = comspec + " /c " + args
                if (GetVersion() >= 0x80000000L or
                        os.path.basename(comspec).lower() == "command.com"):
                    # Win9x, or using command.com on NT. We need to
                    # use the w9xpopen intermediate program. For more
                    # information, see KB Q150956
                    # (http://web.archive.org/web/20011105084002/http://support.microsoft.com/support/kb/articles/Q150/9/56.asp)
                    w9xpopen = self._find_w9xpopen()
                    args = '"%s" %s' % (w9xpopen, args)
                    # Not passing CREATE_NEW_CONSOLE has been known to
                    # cause random failures on win9x.  Specifically a
                    # dialog: "Your program accessed mem currently in
                    # use at xxx" and a hopeful warning about the
                    # stability of your system.  Cost is Ctrl+C wont
                    # kill children.
                    creationflags |= CREATE_NEW_CONSOLE

            # Start the process
            try:
                hp, ht, pid, tid = CreateProcess(executable, args,
                                         # no special security
                                         None, None,
                                         int(not close_fds),
                                         creationflags,
                                         env,
                                         cwd,
                                         startupinfo)
            except pywintypes.error, e:
                # Translate pywintypes.error to WindowsError, which is
                # a subclass of OSError.  FIXME: We should really
                # translate errno using _sys_errlist (or simliar), but
                # how can this be done from Python?
                raise WindowsError(*e.args)

            # Retain the process handle, but close the thread handle
            self._child_created = True
            self._handle = hp
            self.pid = pid
            ht.Close()

            # Child is launched. Close the parent's copy of those pipe
            # handles that only the child should have open.  You need
            # to make sure that no handles to the write end of the
            # output pipe are maintained in this process or else the
            # pipe will not close when the child process exits and the
            # ReadFile will hang.
            if p2cread is not None:
                p2cread.Close()
            if c2pwrite is not None:
                c2pwrite.Close()
            if errwrite is not None:
                errwrite.Close()


        def poll(self, _deadstate=None):
            """Check if child process has terminated.  Returns returncode
            attribute."""
            if self.returncode is None:
                if WaitForSingleObject(self._handle, 0) == WAIT_OBJECT_0:
                    self.returncode = GetExitCodeProcess(self._handle)
            return self.returncode


        def wait(self):
            """Wait for child process to terminate.  Returns returncode
            attribute."""
            if self.returncode is None:
                obj = WaitForSingleObject(self._handle, INFINITE)
                self.returncode = GetExitCodeProcess(self._handle)
            return self.returncode

    elif jython:
        #
        # Jython methods
        #
        def _get_handles(self, stdin, stdout, stderr):
            """Construct and return tuple with IO objects:
            p2cread, p2cwrite, c2pread, c2pwrite, errread, errwrite
            """
            p2cread, p2cwrite = None, None
            c2pread, c2pwrite = None, None
            errread, errwrite = None, None

            if stdin is None:
                pass
            elif stdin == PIPE:
                p2cwrite = PIPE
            elif isinstance(stdin, org.python.core.io.RawIOBase):
                p2cread = stdin
            else:
                # Assuming file-like object
                p2cread = stdin.fileno()

            if stdout is None:
                pass
            elif stdout == PIPE:
                c2pread = PIPE
            elif isinstance(stdout, org.python.core.io.RawIOBase):
                c2pwrite = stdout
            else:
                # Assuming file-like object
                c2pwrite = stdout.fileno()

            if stderr is None:
                pass
            elif stderr == PIPE:
                errread = PIPE
            elif (stderr == STDOUT or
                  isinstance(stderr, org.python.core.io.RawIOBase)):
                errwrite = stderr
            else:
                # Assuming file-like object
                errwrite = stderr.fileno()

            return (p2cread, p2cwrite,
                    c2pread, c2pwrite,
                    errread, errwrite)


        def _stderr_is_stdout(self, errwrite, c2pwrite):
            """Determine if the subprocess' stderr should be redirected
            to stdout
            """
            return (errwrite == STDOUT or c2pwrite not in (None, PIPE) and
                    c2pwrite is errwrite)


        def _coupler_thread(self, *args, **kwargs):
            """Return a _CouplerThread"""
            return _CouplerThread(*args, **kwargs)


        def _setup_env(self, env, builder_env):
            """Carefully merge env with ProcessBuilder's only
            overwriting key/values that differ

            System.getenv (Map<String, String>) may be backed by
            <byte[], byte[]> on UNIX platforms where these are really
            bytes. ProcessBuilder's env inherits its contents and will
            maintain those byte values (which may be butchered as
            Strings) for the subprocess if they haven't been modified.
            """
            # Determine what's safe to merge
            merge_env = dict((key, value) for key, value in env.iteritems()
                             if key not in builder_env or
                             builder_env.get(key) != value)

            # Prune anything not in env
            entries = builder_env.entrySet().iterator()
            for entry in entries:
                if entry.getKey() not in env:
                    entries.remove()

            builder_env.putAll(merge_env)


        def _execute_child(self, args, executable, preexec_fn, close_fds,
                           cwd, env, universal_newlines,
                           startupinfo, creationflags, shell,
                           p2cread, p2cwrite,
                           c2pread, c2pwrite,
                           errread, errwrite):
            """Execute program (Java version)"""

            if isinstance(args, types.StringTypes):
                args = _cmdline2listimpl(args)
            else:
                args = list(args)
                # NOTE: CPython posix (execv) will str() any unicode
                # args first, maybe we should do the same on
                # posix. Windows passes unicode through, however
                if any(not isinstance(arg, (str, unicode)) for arg in args):
                    raise TypeError('args must contain only strings')
            args = _escape_args(args)

            if shell:
                args = _shell_command + args

            if executable is not None:
                args[0] = executable

            builder = java.lang.ProcessBuilder(args)
            # os.environ may be inherited for compatibility with CPython
            self._setup_env(dict(os.environ if env is None else env),
                            builder.environment())

            if cwd is None:
                cwd = os.getcwd()
            elif not os.path.exists(cwd):
                raise OSError(errno.ENOENT, os.strerror(errno.ENOENT), cwd)
            elif not os.path.isdir(cwd):
                raise OSError(errno.ENOTDIR, os.strerror(errno.ENOTDIR), cwd)
            builder.directory(java.io.File(cwd))

            # Let Java manage redirection of stderr to stdout (it's more
            # accurate at doing so than _CouplerThreads). We redirect
            # not only when stderr is marked as STDOUT, but also when
            # c2pwrite is errwrite
            if self._stderr_is_stdout(errwrite, c2pwrite):
                builder.redirectErrorStream(True)

            try:
                self._process = builder.start()
            except (java.io.IOException,
                    java.lang.IllegalArgumentException), e:
                raise OSError(e.getMessage() or e)
            self._child_created = True


        def poll(self, _deadstate=None):
            """Check if child process has terminated.  Returns returncode
            attribute."""
            if self.returncode is None:
                try:
                    self.returncode = self._process.exitValue()
                except java.lang.IllegalThreadStateException:
                    pass
            return self.returncode


        def wait(self):
            """Wait for child process to terminate.  Returns returncode
            attribute."""
            if self.returncode is None:
                self.returncode = self._process.waitFor()
                for coupler in (self._stdout_thread, self._stderr_thread):
                    if coupler:
                        coupler.join()
                if self._stdin_thread:
                    # The stdin thread may be blocked forever, forcibly
                    # stop it
                    self._stdin_thread.interrupt()
            return self.returncode

    else:
        #
        # POSIX methods
        #
        def _get_handles(self, stdin, stdout, stderr):
            """Construct and return tupel with IO objects:
            p2cread, p2cwrite, c2pread, c2pwrite, errread, errwrite
            """
            p2cread, p2cwrite = None, None
            c2pread, c2pwrite = None, None
            errread, errwrite = None, None

            if stdin is None:
                pass
            elif stdin == PIPE:
                p2cread, p2cwrite = os.pipe()
            elif isinstance(stdin, int):
                p2cread = stdin
            else:
                # Assuming file-like object
                p2cread = stdin.fileno()

            if stdout is None:
                pass
            elif stdout == PIPE:
                c2pread, c2pwrite = os.pipe()
            elif isinstance(stdout, int):
                c2pwrite = stdout
            else:
                # Assuming file-like object
                c2pwrite = stdout.fileno()

            if stderr is None:
                pass
            elif stderr == PIPE:
                errread, errwrite = os.pipe()
            elif stderr == STDOUT:
                errwrite = c2pwrite
            elif isinstance(stderr, int):
                errwrite = stderr
            else:
                # Assuming file-like object
                errwrite = stderr.fileno()

            return (p2cread, p2cwrite,
                    c2pread, c2pwrite,
                    errread, errwrite)


        def _set_cloexec_flag(self, fd):
            try:
                cloexec_flag = fcntl.FD_CLOEXEC
            except AttributeError:
                cloexec_flag = 1

            old = fcntl.fcntl(fd, fcntl.F_GETFD)
            fcntl.fcntl(fd, fcntl.F_SETFD, old | cloexec_flag)


        def _close_fds(self, but):
            os.closerange(3, but)
            os.closerange(but + 1, MAXFD)


        def _execute_child(self, args, executable, preexec_fn, close_fds,
                           cwd, env, universal_newlines,
                           startupinfo, creationflags, shell,
                           p2cread, p2cwrite,
                           c2pread, c2pwrite,
                           errread, errwrite):
            """Execute program (POSIX version)"""

            if isinstance(args, types.StringTypes):
                args = [args]
            else:
                args = list(args)

            if shell:
                args = ["/bin/sh", "-c"] + args

            if executable is None:
                executable = args[0]

            # For transferring possible exec failure from child to parent
            # The first char specifies the exception type: 0 means
            # OSError, 1 means some other error.
            errpipe_read, errpipe_write = os.pipe()
            self._set_cloexec_flag(errpipe_write)

            gc_was_enabled = gc.isenabled()
            # Disable gc to avoid bug where gc -> file_dealloc ->
            # write to stderr -> hang.  http://bugs.python.org/issue1336
            gc.disable()
            try:
                self.pid = os.fork()
            except:
                if gc_was_enabled:
                    gc.enable()
                raise
            self._child_created = True
            if self.pid == 0:
                # Child
                try:
                    # Close parent's pipe ends
                    if p2cwrite is not None:
                        os.close(p2cwrite)
                    if c2pread is not None:
                        os.close(c2pread)
                    if errread is not None:
                        os.close(errread)
                    os.close(errpipe_read)

                    # Dup fds for child
                    if p2cread is not None:
                        os.dup2(p2cread, 0)
                    if c2pwrite is not None:
                        os.dup2(c2pwrite, 1)
                    if errwrite is not None:
                        os.dup2(errwrite, 2)

                    # Close pipe fds.  Make sure we don't close the same
                    # fd more than once, or standard fds.
                    if p2cread is not None and p2cread not in (0,):
                        os.close(p2cread)
                    if c2pwrite is not None and c2pwrite not in (p2cread, 1):
                        os.close(c2pwrite)
                    if errwrite is not None and errwrite not in (p2cread, c2pwrite, 2):
                        os.close(errwrite)

                    # Close all other fds, if asked for
                    if close_fds:
                        self._close_fds(but=errpipe_write)

                    if cwd is not None:
                        os.chdir(cwd)

                    if preexec_fn:
                        apply(preexec_fn)

                    if env is None:
                        os.execvp(executable, args)
                    else:
                        os.execvpe(executable, args, env)

                except:
                    exc_type, exc_value, tb = sys.exc_info()
                    # Save the traceback and attach it to the exception object
                    exc_lines = traceback.format_exception(exc_type,
                                                           exc_value,
                                                           tb)
                    exc_value.child_traceback = ''.join(exc_lines)
                    os.write(errpipe_write, pickle.dumps(exc_value))

                # This exitcode won't be reported to applications, so it
                # really doesn't matter what we return.
                os._exit(255)

            # Parent
            if gc_was_enabled:
                gc.enable()
            os.close(errpipe_write)
            if p2cread is not None and p2cwrite is not None:
                os.close(p2cread)
            if c2pwrite is not None and c2pread is not None:
                os.close(c2pwrite)
            if errwrite is not None and errread is not None:
                os.close(errwrite)

            # Wait for exec to fail or succeed; possibly raising exception
            data = os.read(errpipe_read, 1048576) # Exceptions limited to 1 MB
            os.close(errpipe_read)
            if data != "":
                os.waitpid(self.pid, 0)
                child_exception = pickle.loads(data)
                raise child_exception


        def _handle_exitstatus(self, sts):
            if os.WIFSIGNALED(sts):
                self.returncode = -os.WTERMSIG(sts)
            elif os.WIFEXITED(sts):
                self.returncode = os.WEXITSTATUS(sts)
            else:
                # Should never happen
                raise RuntimeError("Unknown child exit status!")


        def poll(self, _deadstate=None):
            """Check if child process has terminated.  Returns returncode
            attribute."""
            if self.returncode is None:
                try:
                    pid, sts = os.waitpid(self.pid, os.WNOHANG)
                    if pid == self.pid:
                        self._handle_exitstatus(sts)
                except os.error:
                    if _deadstate is not None:
                        self.returncode = _deadstate
            return self.returncode


        def wait(self):
            """Wait for child process to terminate.  Returns returncode
            attribute."""
            if self.returncode is None:
                pid, sts = os.waitpid(self.pid, 0)
                self._handle_exitstatus(sts)
            return self.returncode


        def _communicate(self, input):
            read_set = []
            write_set = []
            stdout = None # Return
            stderr = None # Return

            if self.stdin:
                # Flush stdio buffer.  This might block, if the user has
                # been writing to .stdin in an uncontrolled fashion.
                self.stdin.flush()
                if input:
                    write_set.append(self.stdin)
                else:
                    self.stdin.close()
            if self.stdout:
                read_set.append(self.stdout)
                stdout = []
            if self.stderr:
                read_set.append(self.stderr)
                stderr = []

            input_offset = 0
            while read_set or write_set:
                rlist, wlist, xlist = select.select(read_set, write_set, [])

                if self.stdin in wlist:
                    # When select has indicated that the file is writable,
                    # we can write up to PIPE_BUF bytes without risk
                    # blocking.  POSIX defines PIPE_BUF >= 512
                    bytes_written = os.write(self.stdin.fileno(), buffer(input, input_offset, 512))
                    input_offset += bytes_written
                    if input_offset >= len(input):
                        self.stdin.close()
                        write_set.remove(self.stdin)

                if self.stdout in rlist:
                    data = os.read(self.stdout.fileno(), 1024)
                    if data == "":
                        self.stdout.close()
                        read_set.remove(self.stdout)
                    stdout.append(data)

                if self.stderr in rlist:
                    data = os.read(self.stderr.fileno(), 1024)
                    if data == "":
                        self.stderr.close()
                        read_set.remove(self.stderr)
                    stderr.append(data)

            # All data exchanged.  Translate lists into strings.
            if stdout is not None:
                stdout = ''.join(stdout)
            if stderr is not None:
                stderr = ''.join(stderr)

            # Translate newlines, if requested.  We cannot let the file
            # object do the translation: It is based on stdio, which is
            # impossible to combine with select (unless forcing no
            # buffering).
            if self.universal_newlines and hasattr(file, 'newlines'):
                if stdout:
                    stdout = self._translate_newlines(stdout)
                if stderr:
                    stderr = self._translate_newlines(stderr)

            self.wait()
            return (stdout, stderr)


def _demo_posix():
    #
    # Example 1: Simple redirection: Get process list
    #
    plist = Popen(["ps"], stdout=PIPE).communicate()[0]
    print "Process list:"
    print plist

    #
    # Example 2: Change uid before executing child
    #
    if os.getuid() == 0:
        p = Popen(["id"], preexec_fn=lambda: os.setuid(100))
        p.wait()

    #
    # Example 3: Connecting several subprocesses
    #
    print "Looking for 'hda'..."
    p1 = Popen(["dmesg"], stdout=PIPE)
    p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
    print repr(p2.communicate()[0])

    #
    # Example 4: Catch execution error
    #
    print
    print "Trying a weird file..."
    try:
        print Popen(["/this/path/does/not/exist"]).communicate()
    except OSError, e:
        if e.errno == errno.ENOENT:
            print "The file didn't exist.  I thought so..."
            print "Child traceback:"
            print e.child_traceback
        else:
            print "Error", e.errno
    else:
        print >>sys.stderr, "Gosh.  No error."


def _demo_windows():
    #
    # Example 1: Connecting several subprocesses
    #
    print "Looking for 'PROMPT' in set output..."
    p1 = Popen("set", stdout=PIPE, shell=True)
    p2 = Popen('find "PROMPT"', stdin=p1.stdout, stdout=PIPE)
    print repr(p2.communicate()[0])

    #
    # Example 2: Simple execution of program
    #
    print "Executing calc..."
    p = Popen("calc")
    p.wait()


def _demo_jython():
    #
    # Example 1: Return the number of processors on this machine
    #
    print "Running a jython subprocess to return the number of processors..."
    p = Popen([sys.executable, "-c",
               ('import sys;'
                'from java.lang import Runtime;'
                'sys.exit(Runtime.getRuntime().availableProcessors())')])
    print p.wait()

    #
    # Example 2: Connecting several subprocesses
    #
    print "Connecting two jython subprocesses..."
    p1 = Popen([sys.executable, "-c",
                ('import os;'
                 'print os.environ["foo"]')], env=dict(foo='bar'),
               stdout=PIPE)
    p2 = Popen([sys.executable, "-c",
                ('import os, sys;'
                 'their_foo = sys.stdin.read().strip();'
                 'my_foo = os.environ["foo"];'
                 'msg = "Their env\'s foo: %r, My env\'s foo: %r";'
                 'print msg % (their_foo, my_foo)')],
               env=dict(foo='baz'), stdin=p1.stdout, stdout=PIPE)
    print p2.communicate()[0]


if __name__ == "__main__":
    if mswindows:
        _demo_windows()
    elif jython:
        _demo_jython()
    else:
        _demo_posix()