python模块：subprocess

 # subprocess - Subprocesses with accessible I/O streams
 #
 # For more information about this module, see PEP 324.
 #
 # 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"""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.

 For a complete description of this module see the Python documentation.

 Main API
 ========
 run(...): Runs a command, waits for it to complete, then returns a
           CompletedProcess instance.
 Popen(...): A class for flexibly executing a command in a new process

 Constants
 ---------
 DEVNULL: Special value that indicates that os.devnull should be used
 PIPE:    Special value that indicates a pipe should be created
 STDOUT:  Special value that indicates that stderr should go to stdout

 Older API
 =========
 call(...): Runs a command, waits for it to complete, then returns
     the return code.
 check_call(...): Same as call() but raises CalledProcessError()
     if return code is not 0
 check_output(...): Same as check_call() but returns the contents of
     stdout instead of a return code
 getoutput(...): Runs a command in the shell, waits for it to complete,
     then returns the output
 getstatusoutput(...): Runs a command in the shell, waits for it to complete,
     then returns a (exitcode, output) tuple
 """

 import sys
 _mswindows = (sys.platform == "win32")

 import io
 import os
 import time
 import signal
 import builtins
 import warnings
 import errno
 from time import monotonic as _time

 # Exception classes used by this module.
 class SubprocessError(Exception): pass

 class CalledProcessError(SubprocessError):
     """Raised when run() is called with check=True and the process
     returns a non-zero exit status.

     Attributes:
       cmd, returncode, stdout, stderr, output
     """
     def __init__(self, returncode, cmd, output=None, stderr=None):
         self.returncode = returncode
         self.cmd = cmd
         self.output = output
         self.stderr = stderr

     def __str__(self):
         if self.returncode and self.returncode < 0:
             try:
                 return "Command '%s' died with %r." % (
                         self.cmd, signal.Signals(-self.returncode))
             except ValueError:
                 return "Command '%s' died with unknown signal %d." % (
                         self.cmd, -self.returncode)
         else:
             return "Command '%s' returned non-zero exit status %d." % (
                     self.cmd, self.returncode)

     @property
     def stdout(self):
         """Alias for output attribute, to match stderr"""
         return self.output

     @stdout.setter
     def stdout(self, value):
         # There's no obvious reason to set this, but allow it anyway so
         # .stdout is a transparent alias for .output
         self.output = value

 class TimeoutExpired(SubprocessError):
     """This exception is raised when the timeout expires while waiting for a
     child process.

     Attributes:
         cmd, output, stdout, stderr, timeout
     """
     def __init__(self, cmd, timeout, output=None, stderr=None):
         self.cmd = cmd
         self.timeout = timeout
         self.output = output
         self.stderr = stderr

     def __str__(self):
         return ("Command '%s' timed out after %s seconds" %
                 (self.cmd, self.timeout))

     @property
     def stdout(self):
         return self.output

     @stdout.setter
     def stdout(self, value):
         # There's no obvious reason to set this, but allow it anyway so
         # .stdout is a transparent alias for .output
         self.output = value

 if _mswindows:
     import threading
     import msvcrt
     import _winapi
     class STARTUPINFO:
         dwFlags = 0
         hStdInput = None
         hStdOutput = None
         hStdError = None
         wShowWindow = 0
 else:
     import _posixsubprocess
     import select
     import selectors
     try:
         import threading
     except ImportError:
         import dummy_threading as threading

     # When select or poll has indicated that the file is writable,
     # we can write up to _PIPE_BUF bytes without risk of blocking.
     # POSIX defines PIPE_BUF as >= 512.
     _PIPE_BUF = getattr(select, 'PIPE_BUF', 512)

     # poll/select have the advantage of not requiring any extra file
     # descriptor, contrarily to epoll/kqueue (also, they require a single
     # syscall).
     if hasattr(selectors, 'PollSelector'):
         _PopenSelector = selectors.PollSelector
     else:
         _PopenSelector = selectors.SelectSelector

 __all__ = ["Popen", "PIPE", "STDOUT", "call", "check_call", "getstatusoutput",
            "getoutput", "check_output", "run", "CalledProcessError", "DEVNULL",
            "SubprocessError", "TimeoutExpired", "CompletedProcess"]
            # NOTE: We intentionally exclude list2cmdline as it is
            # considered an internal implementation detail.  issue10838.

 if _mswindows:
     from _winapi import (CREATE_NEW_CONSOLE, CREATE_NEW_PROCESS_GROUP,
                          STD_INPUT_HANDLE, STD_OUTPUT_HANDLE,
                          STD_ERROR_HANDLE, SW_HIDE,
                          STARTF_USESTDHANDLES, STARTF_USESHOWWINDOW)

     __all__.extend(["CREATE_NEW_CONSOLE", "CREATE_NEW_PROCESS_GROUP",
                     "STD_INPUT_HANDLE", "STD_OUTPUT_HANDLE",
                     "STD_ERROR_HANDLE", "SW_HIDE",
                     "STARTF_USESTDHANDLES", "STARTF_USESHOWWINDOW",
                     "STARTUPINFO"])

     class Handle(int):
         closed = False

         def Close(self, CloseHandle=_winapi.CloseHandle):
             if not self.closed:
                 self.closed = True
                 CloseHandle(self)

         def Detach(self):
             if not self.closed:
                 self.closed = True
                 return int(self)
             raise ValueError("already closed")

         def __repr__(self):
             return "%s(%d)" % (self.__class__.__name__, int(self))

         __del__ = Close
         __str__ = __repr__

 # This lists holds Popen instances for which the underlying process had not
 # exited at the time its __del__ method got called: those processes are wait()ed
 # for synchronously from _cleanup() when a new Popen object is created, to avoid
 # zombie processes.
 _active = []

 def _cleanup():
     for inst in _active[:]:
         res = inst._internal_poll(_deadstate=sys.maxsize)
         if res is not None:
             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
 DEVNULL = -3

 # XXX This function is only used by multiprocessing and the test suite,
 # but it's here so that it can be imported when Python is compiled without
 # threads.

 def _optim_args_from_interpreter_flags():
     """Return a list of command-line arguments reproducing the current
     optimization settings in sys.flags."""
     args = []
     value = sys.flags.optimize
     if value > 0:
         args.append('-' + 'O' * value)
     return args

 def _args_from_interpreter_flags():
     """Return a list of command-line arguments reproducing the current
     settings in sys.flags and sys.warnoptions."""
     flag_opt_map = {
         'debug': 'd',
         # 'inspect': 'i',
         # 'interactive': 'i',
         'dont_write_bytecode': 'B',
         'no_user_site': 's',
         'no_site': 'S',
         'ignore_environment': 'E',
         'verbose': 'v',
         'bytes_warning': 'b',
         'quiet': 'q',
         # -O is handled in _optim_args_from_interpreter_flags()
     }
     args = _optim_args_from_interpreter_flags()
     for flag, opt in flag_opt_map.items():
         v = getattr(sys.flags, flag)
         if v > 0:
             args.append('-' + opt * v)
     for opt in sys.warnoptions:
         args.append('-W' + opt)
     return args

 def call(*popenargs, timeout=None, **kwargs):
     """Run command with arguments.  Wait for command to complete or
     timeout, then return the returncode attribute.

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

     retcode = call(["ls", "-l"])
     """
     with Popen(*popenargs, **kwargs) as p:
         try:
             return p.wait(timeout=timeout)
         except:
             p.kill()
             p.wait()
             raise

 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 call function.  Example:

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

 def check_output(*popenargs, timeout=None, **kwargs):
     r"""Run command with arguments and return its output.

     If the exit code was non-zero it raises a CalledProcessError.  The
     CalledProcessError object will have the return code in the returncode
     attribute and output in the output attribute.

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

     >>> check_output(["ls", "-l", "/dev/null"])
     b'crw-rw-rw- 1 root root 1, 3 Oct 18  2007 /dev/null\n'

     The stdout argument is not allowed as it is used internally.
     To capture standard error in the result, use stderr=STDOUT.

     >>> check_output(["/bin/sh", "-c",
     ...               "ls -l non_existent_file ; exit 0"],
     ...              stderr=STDOUT)
     b'ls: non_existent_file: No such file or directory\n'

     There is an additional optional argument, "input", allowing you to
     pass a string to the subprocess's stdin.  If you use this argument
     you may not also use the Popen constructor's "stdin" argument, as
     it too will be used internally.  Example:

     >>> check_output(["sed", "-e", "s/foo/bar/"],
     ...              input=b"when in the course of fooman events\n")
     b'when in the course of barman events\n'

     If universal_newlines=True is passed, the "input" argument must be a
     string and the return value will be a string rather than bytes.
     """
     if 'stdout' in kwargs:
         raise ValueError('stdout argument not allowed, it will be overridden.')

     if 'input' in kwargs and kwargs['input'] is None:
         # Explicitly passing input=None was previously equivalent to passing an
         # empty string. That is maintained here for backwards compatibility.
         kwargs['input'] = '' if kwargs.get('universal_newlines', False) else b''

     return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
                **kwargs).stdout

 class CompletedProcess(object):
     """A process that has finished running.

     This is returned by run().

     Attributes:
       args: The list or str args passed to run().
       returncode: The exit code of the process, negative for signals.
       stdout: The standard output (None if not captured).
       stderr: The standard error (None if not captured).
     """
     def __init__(self, args, returncode, stdout=None, stderr=None):
         self.args = args
         self.returncode = returncode
         self.stdout = stdout
         self.stderr = stderr

     def __repr__(self):
         args = ['args={!r}'.format(self.args),
                 'returncode={!r}'.format(self.returncode)]
         if self.stdout is not None:
             args.append('stdout={!r}'.format(self.stdout))
         if self.stderr is not None:
             args.append('stderr={!r}'.format(self.stderr))
         return "{}({})".format(type(self).__name__, ', '.join(args))

     def check_returncode(self):
         """Raise CalledProcessError if the exit code is non-zero."""
         if self.returncode:
             raise CalledProcessError(self.returncode, self.args, self.stdout,
                                      self.stderr)

 def run(*popenargs, input=None, timeout=None, check=False, **kwargs):
     """Run command with arguments and return a CompletedProcess instance.

     The returned instance will have attributes args, returncode, stdout and
     stderr. By default, stdout and stderr are not captured, and those attributes
     will be None. Pass stdout=PIPE and/or stderr=PIPE in order to capture them.

     If check is True and the exit code was non-zero, it raises a
     CalledProcessError. The CalledProcessError object will have the return code
     in the returncode attribute, and output & stderr attributes if those streams
     were captured.

     If timeout is given, and the process takes too long, a TimeoutExpired
     exception will be raised.

     There is an optional argument "input", allowing you to
     pass a string to the subprocess's stdin.  If you use this argument
     you may not also use the Popen constructor's "stdin" argument, as
     it will be used internally.

     The other arguments are the same as for the Popen constructor.

     If universal_newlines=True is passed, the "input" argument must be a
     string and stdout/stderr in the returned object will be strings rather than
     bytes.
     """
     if input is not None:
         if 'stdin' in kwargs:
             raise ValueError('stdin and input arguments may not both be used.')
         kwargs['stdin'] = PIPE

     with Popen(*popenargs, **kwargs) as process:
         try:
             stdout, stderr = process.communicate(input, timeout=timeout)
         except TimeoutExpired:
             process.kill()
             stdout, stderr = process.communicate()
             raise TimeoutExpired(process.args, timeout, output=stdout,
                                  stderr=stderr)
         except:
             process.kill()
             process.wait()
             raise
         retcode = process.poll()
         if check and retcode:
             raise CalledProcessError(retcode, process.args,
                                      output=stdout, stderr=stderr)
     return CompletedProcess(process.args, retcode, stdout, stderr)

 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
        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/en-us/library/17w5ykft.aspx
     # or search http://msdn.microsoft.com for
     # "Parsing C++ Command-Line Arguments"
     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 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)

 # Various tools for executing commands and looking at their output and status.
 #

 def getstatusoutput(cmd):
     """Return (exitcode, output) of executing cmd in a shell.

     Execute the string 'cmd' in a shell with 'check_output' and
     return a 2-tuple (status, output). The locale encoding is used
     to decode the output and process newlines.

     A trailing newline is stripped from the output.
     The exit status for the command can be interpreted
     according to the rules for the function 'wait'. Example:

     >>> import subprocess
     >>> subprocess.getstatusoutput('ls /bin/ls')
     (0, '/bin/ls')
     >>> subprocess.getstatusoutput('cat /bin/junk')
     (1, 'cat: /bin/junk: No such file or directory')
     >>> subprocess.getstatusoutput('/bin/junk')
     (127, 'sh: /bin/junk: not found')
     >>> subprocess.getstatusoutput('/bin/kill $$')
     (-15, '')
     """
     try:
         data = check_output(cmd, shell=True, universal_newlines=True, stderr=STDOUT)
         exitcode = 0
     except CalledProcessError as ex:
         data = ex.output
         exitcode = ex.returncode
     if data[-1:] == '\n':
         data = data[:-1]
     return exitcode, data

 def getoutput(cmd):
     """Return output (stdout or stderr) of executing cmd in a shell.

     Like getstatusoutput(), except the exit status is ignored and the return
     value is a string containing the command's output.  Example:

     >>> import subprocess
     >>> subprocess.getoutput('ls /bin/ls')
     '/bin/ls'
     """
     return getstatusoutput(cmd)[1]

 _PLATFORM_DEFAULT_CLOSE_FDS = object()

 class Popen(object):
     """ Execute a child program in a new process.

     For a complete description of the arguments see the Python documentation.

     Arguments:
       args: A string, or a sequence of program arguments.

       bufsize: supplied as the buffering argument to the open() function when
           creating the stdin/stdout/stderr pipe file objects

       executable: A replacement program to execute.

       stdin, stdout and stderr: These specify the executed programs' standard
           input, standard output and standard error file handles, respectively.

       preexec_fn: (POSIX only) An object to be called in the child process
           just before the child is executed.

       close_fds: Controls closing or inheriting of file descriptors.

       shell: If true, the command will be executed through the shell.

       cwd: Sets the current directory before the child is executed.

       env: Defines the environment variables for the new process.

       universal_newlines: If true, use universal line endings for file
           objects stdin, stdout and stderr.

       startupinfo and creationflags (Windows only)

       restore_signals (POSIX only)

       start_new_session (POSIX only)

       pass_fds (POSIX only)

       encoding and errors: Text mode encoding and error handling to use for
           file objects stdin, stdout and stderr.

     Attributes:
         stdin, stdout, stderr, pid, returncode
     """
     _child_created = False  # Set here since __del__ checks it

     def __init__(self, args, bufsize=-1, executable=None,
                  stdin=None, stdout=None, stderr=None,
                  preexec_fn=None, close_fds=_PLATFORM_DEFAULT_CLOSE_FDS,
                  shell=False, cwd=None, env=None, universal_newlines=False,
                  startupinfo=None, creationflags=0,
                  restore_signals=True, start_new_session=False,
                  pass_fds=(), *, encoding=None, errors=None):
         """Create new Popen instance."""
         _cleanup()
         # Held while anything is calling waitpid before returncode has been
         # updated to prevent clobbering returncode if wait() or poll() are
         # called from multiple threads at once.  After acquiring the lock,
         # code must re-check self.returncode to see if another thread just
         # finished a waitpid() call.
         self._waitpid_lock = threading.Lock()

         self._input = None
         self._communication_started = False
         if bufsize is None:
             bufsize = -1  # Restore default
         if not isinstance(bufsize, int):
             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")
             any_stdio_set = (stdin is not None or stdout is not None or
                              stderr is not None)
             if close_fds is _PLATFORM_DEFAULT_CLOSE_FDS:
                 if any_stdio_set:
                     close_fds = False
                 else:
                     close_fds = True
             elif close_fds and any_stdio_set:
                 raise ValueError(
                         "close_fds is not supported on Windows platforms"
                         " if you redirect stdin/stdout/stderr")
         else:
             # POSIX
             if close_fds is _PLATFORM_DEFAULT_CLOSE_FDS:
                 close_fds = True
             if pass_fds and not close_fds:
                 warnings.warn("pass_fds overriding close_fds.", RuntimeWarning)
                 close_fds = True
             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")

         self.args = args
         self.stdin = None
         self.stdout = None
         self.stderr = None
         self.pid = None
         self.returncode = None
         self.universal_newlines = universal_newlines
         self.encoding = encoding
         self.errors = errors

         # 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 -1 when not using PIPEs. The child objects are -1
         # when not redirecting.

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

         # We wrap OS handles *before* launching the child, otherwise a
         # quickly terminating child could make our fds unwrappable
         # (see #8458).

         if _mswindows:
             if p2cwrite != -1:
                 p2cwrite = msvcrt.open_osfhandle(p2cwrite.Detach(), 0)
             if c2pread != -1:
                 c2pread = msvcrt.open_osfhandle(c2pread.Detach(), 0)
             if errread != -1:
                 errread = msvcrt.open_osfhandle(errread.Detach(), 0)

         text_mode = encoding or errors or universal_newlines

         self._closed_child_pipe_fds = False

         try:
             if p2cwrite != -1:
                 self.stdin = io.open(p2cwrite, 'wb', bufsize)
                 if text_mode:
                     self.stdin = io.TextIOWrapper(self.stdin, write_through=True,
                             line_buffering=(bufsize == 1),
                             encoding=encoding, errors=errors)
             if c2pread != -1:
                 self.stdout = io.open(c2pread, 'rb', bufsize)
                 if text_mode:
                     self.stdout = io.TextIOWrapper(self.stdout,
                             encoding=encoding, errors=errors)
             if errread != -1:
                 self.stderr = io.open(errread, 'rb', bufsize)
                 if text_mode:
                     self.stderr = io.TextIOWrapper(self.stderr,
                             encoding=encoding, errors=errors)

             self._execute_child(args, executable, preexec_fn, close_fds,
                                 pass_fds, cwd, env,
                                 startupinfo, creationflags, shell,
                                 p2cread, p2cwrite,
                                 c2pread, c2pwrite,
                                 errread, errwrite,
                                 restore_signals, start_new_session)
         except:
             # Cleanup if the child failed starting.
             for f in filter(None, (self.stdin, self.stdout, self.stderr)):
                 try:
                     f.close()
                 except OSError:
                     pass  # Ignore EBADF or other errors.

             if not self._closed_child_pipe_fds:
                 to_close = []
                 if stdin == PIPE:
                     to_close.append(p2cread)
                 if stdout == PIPE:
                     to_close.append(c2pwrite)
                 if stderr == PIPE:
                     to_close.append(errwrite)
                 if hasattr(self, '_devnull'):
                     to_close.append(self._devnull)
                 for fd in to_close:
                     try:
                         if _mswindows and isinstance(fd, Handle):
                             fd.Close()
                         else:
                             os.close(fd)
                     except OSError:
                         pass

             raise

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

     def __enter__(self):
         return self

     def __exit__(self, type, value, traceback):
         if self.stdout:
             self.stdout.close()
         if self.stderr:
             self.stderr.close()
         try:  # Flushing a BufferedWriter may raise an error
             if self.stdin:
                 self.stdin.close()
         finally:
             # Wait for the process to terminate, to avoid zombies.
             self.wait()

     def __del__(self, _maxsize=sys.maxsize, _warn=warnings.warn):
         if not self._child_created:
             # We didn't get to successfully create a child process.
             return
         if self.returncode is None:
             # Not reading subprocess exit status creates a zombi process which
             # is only destroyed at the parent python process exit
             _warn("subprocess %s is still running" % self.pid,
                   ResourceWarning, source=self)
         # In case the child hasn't been waited on, check if it's done.
         self._internal_poll(_deadstate=_maxsize)
         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 _get_devnull(self):
         if not hasattr(self, '_devnull'):
             self._devnull = os.open(os.devnull, os.O_RDWR)
         return self._devnull

     def _stdin_write(self, input):
         if input:
             try:
                 self.stdin.write(input)
             except BrokenPipeError:
                 pass  # communicate() must ignore broken pipe errors.
             except OSError as exc:
                 if exc.errno == errno.EINVAL:
                     # bpo-19612, bpo-30418: On Windows, stdin.write() fails
                     # with EINVAL if the child process exited or if the child
                     # process is still running but closed the pipe.
                     pass
                 else:
                     raise

         try:
             self.stdin.close()
         except BrokenPipeError:
             pass  # communicate() must ignore broken pipe errors.
         except OSError as exc:
             if exc.errno == errno.EINVAL:
                 pass
             else:
                 raise

     def communicate(self, input=None, timeout=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 data to be sent to the
         child process (if self.universal_newlines is True, this should
         be a string; if it is False, "input" should be bytes), or
         None, if no data should be sent to the child.

         communicate() returns a tuple (stdout, stderr).  These will be
         bytes or, if self.universal_newlines was True, a string.
         """

         if self._communication_started and input:
             raise ValueError("Cannot send input after starting communication")

         # Optimization: If we are not worried about timeouts, we haven't
         # started communicating, and we have one or zero pipes, using select()
         # or threads is unnecessary.
         if (timeout is None and not self._communication_started and
             [self.stdin, self.stdout, self.stderr].count(None) >= 2):
             stdout = None
             stderr = None
             if self.stdin:
                 self._stdin_write(input)
             elif self.stdout:
                 stdout = self.stdout.read()
                 self.stdout.close()
             elif self.stderr:
                 stderr = self.stderr.read()
                 self.stderr.close()
             self.wait()
         else:
             if timeout is not None:
                 endtime = _time() + timeout
             else:
                 endtime = None

             try:
                 stdout, stderr = self._communicate(input, endtime, timeout)
             finally:
                 self._communication_started = True

             sts = self.wait(timeout=self._remaining_time(endtime))

         return (stdout, stderr)

     def poll(self):
         """Check if child process has terminated. Set and return returncode
         attribute."""
         return self._internal_poll()

     def _remaining_time(self, endtime):
         """Convenience for _communicate when computing timeouts."""
         if endtime is None:
             return None
         else:
             return endtime - _time()

     def _check_timeout(self, endtime, orig_timeout):
         """Convenience for checking if a timeout has expired."""
         if endtime is None:
             return
         if _time() > endtime:
             raise TimeoutExpired(self.args, orig_timeout)

     if _mswindows:
         #
         # Windows methods
         #
         def _get_handles(self, stdin, stdout, stderr):
             """Construct and return tuple with IO objects:
             p2cread, p2cwrite, c2pread, c2pwrite, errread, errwrite
             """
             if stdin is None and stdout is None and stderr is None:
                 return (-1, -1, -1, -1, -1, -1)

             p2cread, p2cwrite = -1, -1
             c2pread, c2pwrite = -1, -1
             errread, errwrite = -1, -1

             if stdin is None:
                 p2cread = _winapi.GetStdHandle(_winapi.STD_INPUT_HANDLE)
                 if p2cread is None:
                     p2cread, _ = _winapi.CreatePipe(None, 0)
                     p2cread = Handle(p2cread)
                     _winapi.CloseHandle(_)
             elif stdin == PIPE:
                 p2cread, p2cwrite = _winapi.CreatePipe(None, 0)
                 p2cread, p2cwrite = Handle(p2cread), Handle(p2cwrite)
             elif stdin == DEVNULL:
                 p2cread = msvcrt.get_osfhandle(self._get_devnull())
             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 = _winapi.GetStdHandle(_winapi.STD_OUTPUT_HANDLE)
                 if c2pwrite is None:
                     _, c2pwrite = _winapi.CreatePipe(None, 0)
                     c2pwrite = Handle(c2pwrite)
                     _winapi.CloseHandle(_)
             elif stdout == PIPE:
                 c2pread, c2pwrite = _winapi.CreatePipe(None, 0)
                 c2pread, c2pwrite = Handle(c2pread), Handle(c2pwrite)
             elif stdout == DEVNULL:
                 c2pwrite = msvcrt.get_osfhandle(self._get_devnull())
             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 = _winapi.GetStdHandle(_winapi.STD_ERROR_HANDLE)
                 if errwrite is None:
                     _, errwrite = _winapi.CreatePipe(None, 0)
                     errwrite = Handle(errwrite)
                     _winapi.CloseHandle(_)
             elif stderr == PIPE:
                 errread, errwrite = _winapi.CreatePipe(None, 0)
                 errread, errwrite = Handle(errread), Handle(errwrite)
             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:
                 # 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"""
             h = _winapi.DuplicateHandle(
                 _winapi.GetCurrentProcess(), handle,
                 _winapi.GetCurrentProcess(), 0, 1,
                 _winapi.DUPLICATE_SAME_ACCESS)
             return Handle(h)

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

             assert not pass_fds, "pass_fds not supported on Windows."

             if not isinstance(args, str):
                 args = list2cmdline(args)

             # Process startup details
             if startupinfo is None:
                 startupinfo = STARTUPINFO()
             if -1 not in (p2cread, c2pwrite, errwrite):
                 startupinfo.dwFlags |= _winapi.STARTF_USESTDHANDLES
                 startupinfo.hStdInput = p2cread
                 startupinfo.hStdOutput = c2pwrite
                 startupinfo.hStdError = errwrite

             if shell:
                 startupinfo.dwFlags |= _winapi.STARTF_USESHOWWINDOW
                 startupinfo.wShowWindow = _winapi.SW_HIDE
                 comspec = os.environ.get("COMSPEC", "cmd.exe")
                 args = '{} /c "{}"'.format (comspec, args)

             # Start the process
             try:
                 hp, ht, pid, tid = _winapi.CreateProcess(executable, args,
                                          # no special security
                                          None, None,
                                          int(not close_fds),
                                          creationflags,
                                          env,
                                          os.fspath(cwd) if cwd is not None else None,
                                          startupinfo)
             finally:
                 # 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 != -1:
                     p2cread.Close()
                 if c2pwrite != -1:
                     c2pwrite.Close()
                 if errwrite != -1:
                     errwrite.Close()
                 if hasattr(self, '_devnull'):
                     os.close(self._devnull)
                 # Prevent a double close of these handles/fds from __init__
                 # on error.
                 self._closed_child_pipe_fds = True

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

         def _internal_poll(self, _deadstate=None,
                 _WaitForSingleObject=_winapi.WaitForSingleObject,
                 _WAIT_OBJECT_0=_winapi.WAIT_OBJECT_0,
                 _GetExitCodeProcess=_winapi.GetExitCodeProcess):
             """Check if child process has terminated.  Returns returncode
             attribute.

             This method is called by __del__, so it can only refer to objects
             in its local scope.

             """
             if self.returncode is None:
                 if _WaitForSingleObject(self._handle, 0) == _WAIT_OBJECT_0:
                     self.returncode = _GetExitCodeProcess(self._handle)
             return self.returncode

         def wait(self, timeout=None, endtime=None):
             """Wait for child process to terminate.  Returns returncode
             attribute."""
             if endtime is not None:
                 warnings.warn(
                     "'endtime' argument is deprecated; use 'timeout'.",
                     DeprecationWarning,
                     stacklevel=2)
                 timeout = self._remaining_time(endtime)
             if timeout is None:
                 timeout_millis = _winapi.INFINITE
             else:
                 timeout_millis = int(timeout * 1000)
             if self.returncode is None:
                 result = _winapi.WaitForSingleObject(self._handle,
                                                     timeout_millis)
                 if result == _winapi.WAIT_TIMEOUT:
                     raise TimeoutExpired(self.args, timeout)
                 self.returncode = _winapi.GetExitCodeProcess(self._handle)
             return self.returncode

         def _readerthread(self, fh, buffer):
             buffer.append(fh.read())
             fh.close()

         def _communicate(self, input, endtime, orig_timeout):
             # Start reader threads feeding into a list hanging off of this
             # object, unless they've already been started.
             if self.stdout and not hasattr(self, "_stdout_buff"):
                 self._stdout_buff = []
                 self.stdout_thread = \
                         threading.Thread(target=self._readerthread,
                                          args=(self.stdout, self._stdout_buff))
                 self.stdout_thread.daemon = True
                 self.stdout_thread.start()
             if self.stderr and not hasattr(self, "_stderr_buff"):
                 self._stderr_buff = []
                 self.stderr_thread = \
                         threading.Thread(target=self._readerthread,
                                          args=(self.stderr, self._stderr_buff))
                 self.stderr_thread.daemon = True
                 self.stderr_thread.start()

             if self.stdin:
                 self._stdin_write(input)

             # Wait for the reader threads, or time out.  If we time out, the
             # threads remain reading and the fds left open in case the user
             # calls communicate again.
             if self.stdout is not None:
                 self.stdout_thread.join(self._remaining_time(endtime))
                 if self.stdout_thread.is_alive():
                     raise TimeoutExpired(self.args, orig_timeout)
             if self.stderr is not None:
                 self.stderr_thread.join(self._remaining_time(endtime))
                 if self.stderr_thread.is_alive():
                     raise TimeoutExpired(self.args, orig_timeout)

             # Collect the output from and close both pipes, now that we know
             # both have been read successfully.
             stdout = None
             stderr = None
             if self.stdout:
                 stdout = self._stdout_buff
                 self.stdout.close()
             if self.stderr:
                 stderr = self._stderr_buff
                 self.stderr.close()

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

             return (stdout, stderr)

         def send_signal(self, sig):
             """Send a signal to the process."""
             # Don't signal a process that we know has already died.
             if self.returncode is not None:
                 return
             if sig == signal.SIGTERM:
                 self.terminate()
             elif sig == signal.CTRL_C_EVENT:
                 os.kill(self.pid, signal.CTRL_C_EVENT)
             elif sig == signal.CTRL_BREAK_EVENT:
                 os.kill(self.pid, signal.CTRL_BREAK_EVENT)
             else:
                 raise ValueError("Unsupported signal: {}".format(sig))

         def terminate(self):
             """Terminates the process."""
             # Don't terminate a process that we know has already died.
             if self.returncode is not None:
                 return
             try:
                 _winapi.TerminateProcess(self._handle, 1)
             except PermissionError:
                 # ERROR_ACCESS_DENIED (winerror 5) is received when the
                 # process already died.
                 rc = _winapi.GetExitCodeProcess(self._handle)
                 if rc == _winapi.STILL_ACTIVE:
                     raise
                 self.returncode = rc

         kill = terminate

     else:
         #
         # POSIX methods
         #
         def _get_handles(self, stdin, stdout, stderr):
             """Construct and return tuple with IO objects:
             p2cread, p2cwrite, c2pread, c2pwrite, errread, errwrite
             """
             p2cread, p2cwrite = -1, -1
             c2pread, c2pwrite = -1, -1
             errread, errwrite = -1, -1

             if stdin is None:
                 pass
             elif stdin == PIPE:
                 p2cread, p2cwrite = os.pipe()
             elif stdin == DEVNULL:
                 p2cread = self._get_devnull()
             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 stdout == DEVNULL:
                 c2pwrite = self._get_devnull()
             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:
                 if c2pwrite != -1:
                     errwrite = c2pwrite
                 else: # child's stdout is not set, use parent's stdout
                     errwrite = sys.__stdout__.fileno()
             elif stderr == DEVNULL:
                 errwrite = self._get_devnull()
             elif isinstance(stderr, int):
                 errwrite = stderr
             else:
                 # Assuming file-like object
                 errwrite = stderr.fileno()

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

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

             if isinstance(args, (str, bytes)):
                 args = [args]
             else:
                 args = list(args)

             if shell:
                 args = ["/bin/sh", "-c"] + args
                 if executable:
                     args[0] = executable

             if executable is None:
                 executable = args[0]
             orig_executable = executable

             # For transferring possible exec failure from child to parent.
             # Data format: "exception name:hex errno:description"
             # Pickle is not used; it is complex and involves memory allocation.
             errpipe_read, errpipe_write = os.pipe()
             # errpipe_write must not be in the standard io 0, 1, or 2 fd range.
             low_fds_to_close = []
             while errpipe_write < 3:
                 low_fds_to_close.append(errpipe_write)
                 errpipe_write = os.dup(errpipe_write)
             for low_fd in low_fds_to_close:
                 os.close(low_fd)
             try:
                 try:
                     # We must avoid complex work that could involve
                     # malloc or free in the child process to avoid
                     # potential deadlocks, thus we do all this here.
                     # and pass it to fork_exec()

                     if env is not None:
                         env_list = []
                         for k, v in env.items():
                             k = os.fsencode(k)
                             if b'=' in k:
                                 raise ValueError("illegal environment variable name")
                             env_list.append(k + b'=' + os.fsencode(v))
                     else:
                         env_list = None  # Use execv instead of execve.
                     executable = os.fsencode(executable)
                     if os.path.dirname(executable):
                         executable_list = (executable,)
                     else:
                         # This matches the behavior of os._execvpe().
                         executable_list = tuple(
                             os.path.join(os.fsencode(dir), executable)
                             for dir in os.get_exec_path(env))
                     fds_to_keep = set(pass_fds)
                     fds_to_keep.add(errpipe_write)
                     self.pid = _posixsubprocess.fork_exec(
                             args, executable_list,
                             close_fds, tuple(sorted(map(int, fds_to_keep))),
                             cwd, env_list,
                             p2cread, p2cwrite, c2pread, c2pwrite,
                             errread, errwrite,
                             errpipe_read, errpipe_write,
                             restore_signals, start_new_session, preexec_fn)
                     self._child_created = True
                 finally:
                     # be sure the FD is closed no matter what
                     os.close(errpipe_write)

                 # self._devnull is not always defined.
                 devnull_fd = getattr(self, '_devnull', None)
                 if p2cread != -1 and p2cwrite != -1 and p2cread != devnull_fd:
                     os.close(p2cread)
                 if c2pwrite != -1 and c2pread != -1 and c2pwrite != devnull_fd:
                     os.close(c2pwrite)
                 if errwrite != -1 and errread != -1 and errwrite != devnull_fd:
                     os.close(errwrite)
                 if devnull_fd is not None:
                     os.close(devnull_fd)
                 # Prevent a double close of these fds from __init__ on error.
                 self._closed_child_pipe_fds = True

                 # Wait for exec to fail or succeed; possibly raising an
                 # exception (limited in size)
                 errpipe_data = bytearray()
                 while True:
                     part = os.read(errpipe_read, 50000)
                     errpipe_data += part
                     if not part or len(errpipe_data) > 50000:
                         break
             finally:
                 # be sure the FD is closed no matter what
                 os.close(errpipe_read)

             if errpipe_data:
                 try:
                     pid, sts = os.waitpid(self.pid, 0)
                     if pid == self.pid:
                         self._handle_exitstatus(sts)
                     else:
                         self.returncode = sys.maxsize
                 except ChildProcessError:
                     pass

                 try:
                     exception_name, hex_errno, err_msg = (
                             errpipe_data.split(b':', 2))
                     # The encoding here should match the encoding
                     # written in by the subprocess implementations
                     # like _posixsubprocess
                     err_msg = err_msg.decode()
                 except ValueError:
                     exception_name = b'SubprocessError'
                     hex_errno = b''
                     err_msg = 'Bad exception data from child: {!r}'.format(
                                   bytes(errpipe_data))
                 child_exception_type = getattr(
                         builtins, exception_name.decode('ascii'),
                         SubprocessError)
                 if issubclass(child_exception_type, OSError) and hex_errno:
                     errno_num = int(hex_errno, 16)
                     child_exec_never_called = (err_msg == "noexec")
                     if child_exec_never_called:
                         err_msg = ""
                         # The error must be from chdir(cwd).
                         err_filename = cwd
                     else:
                         err_filename = orig_executable
                     if errno_num != 0:
                         err_msg = os.strerror(errno_num)
                         if errno_num == errno.ENOENT:
                             err_msg += ': ' + repr(err_filename)
                     raise child_exception_type(errno_num, err_msg, err_filename)
                 raise child_exception_type(err_msg)

         def _handle_exitstatus(self, sts, _WIFSIGNALED=os.WIFSIGNALED,
                 _WTERMSIG=os.WTERMSIG, _WIFEXITED=os.WIFEXITED,
                 _WEXITSTATUS=os.WEXITSTATUS, _WIFSTOPPED=os.WIFSTOPPED,
                 _WSTOPSIG=os.WSTOPSIG):
             """All callers to this function MUST hold self._waitpid_lock."""
             # This method is called (indirectly) by __del__, so it cannot
             # refer to anything outside of its local scope.
             if _WIFSIGNALED(sts):
                 self.returncode = -_WTERMSIG(sts)
             elif _WIFEXITED(sts):
                 self.returncode = _WEXITSTATUS(sts)
             elif _WIFSTOPPED(sts):
                 self.returncode = -_WSTOPSIG(sts)
             else:
                 # Should never happen
                 raise SubprocessError("Unknown child exit status!")

         def _internal_poll(self, _deadstate=None, _waitpid=os.waitpid,
                 _WNOHANG=os.WNOHANG, _ECHILD=errno.ECHILD):
             """Check if child process has terminated.  Returns returncode
             attribute.

             This method is called by __del__, so it cannot reference anything
             outside of the local scope (nor can any methods it calls).

             """
             if self.returncode is None:
                 if not self._waitpid_lock.acquire(False):
                     # Something else is busy calling waitpid.  Don't allow two
                     # at once.  We know nothing yet.
                     return None
                 try:
                     if self.returncode is not None:
                         return self.returncode  # Another thread waited.
                     pid, sts = _waitpid(self.pid, _WNOHANG)
                     if pid == self.pid:
                         self._handle_exitstatus(sts)
                 except OSError as e:
                     if _deadstate is not None:
                         self.returncode = _deadstate
                     elif e.errno == _ECHILD:
                         # This happens if SIGCLD is set to be ignored or
                         # waiting for child processes has otherwise been
                         # disabled for our process.  This child is dead, we
                         # can't get the status.
                         # http://bugs.python.org/issue15756
                         self.returncode = 0
                 finally:
                     self._waitpid_lock.release()
             return self.returncode

         def _try_wait(self, wait_flags):
             """All callers to this function MUST hold self._waitpid_lock."""
             try:
                 (pid, sts) = os.waitpid(self.pid, wait_flags)
             except ChildProcessError:
                 # This happens if SIGCLD is set to be ignored or waiting
                 # for child processes has otherwise been disabled for our
                 # process.  This child is dead, we can't get the status.
                 pid = self.pid
                 sts = 0
             return (pid, sts)

         def wait(self, timeout=None, endtime=None):
             """Wait for child process to terminate.  Returns returncode
             attribute."""
             if self.returncode is not None:
                 return self.returncode

             if endtime is not None:
                 warnings.warn(
                     "'endtime' argument is deprecated; use 'timeout'.",
                     DeprecationWarning,
                     stacklevel=2)
             if endtime is not None or timeout is not None:
                 if endtime is None:
                     endtime = _time() + timeout
                 elif timeout is None:
                     timeout = self._remaining_time(endtime)

             if endtime is not None:
                 # Enter a busy loop if we have a timeout.  This busy loop was
                 # cribbed from Lib/threading.py in Thread.wait() at r71065.
                 delay = 0.0005 # 500 us -> initial delay of 1 ms
                 while True:
                     if self._waitpid_lock.acquire(False):
                         try:
                             if self.returncode is not None:
                                 break  # Another thread waited.
                             (pid, sts) = self._try_wait(os.WNOHANG)
                             assert pid == self.pid or pid == 0
                             if pid == self.pid:
                                 self._handle_exitstatus(sts)
                                 break
                         finally:
                             self._waitpid_lock.release()
                     remaining = self._remaining_time(endtime)
                     if remaining <= 0:
                         raise TimeoutExpired(self.args, timeout)
                     delay = min(delay * 2, remaining, .05)
                     time.sleep(delay)
             else:
                 while self.returncode is None:
                     with self._waitpid_lock:
                         if self.returncode is not None:
                             break  # Another thread waited.
                         (pid, sts) = self._try_wait(0)
                         # Check the pid and loop as waitpid has been known to
                         # return 0 even without WNOHANG in odd situations.
                         # http://bugs.python.org/issue14396.
                         if pid == self.pid:
                             self._handle_exitstatus(sts)
             return self.returncode

         def _communicate(self, input, endtime, orig_timeout):
             if self.stdin and not self._communication_started:
                 # Flush stdio buffer.  This might block, if the user has
                 # been writing to .stdin in an uncontrolled fashion.
                 try:
                     self.stdin.flush()
                 except BrokenPipeError:
                     pass  # communicate() must ignore BrokenPipeError.
                 if not input:
                     try:
                         self.stdin.close()
                     except BrokenPipeError:
                         pass  # communicate() must ignore BrokenPipeError.

             stdout = None
             stderr = None

             # Only create this mapping if we haven't already.
             if not self._communication_started:
                 self._fileobj2output = {}
                 if self.stdout:
                     self._fileobj2output[self.stdout] = []
                 if self.stderr:
                     self._fileobj2output[self.stderr] = []

             if self.stdout:
                 stdout = self._fileobj2output[self.stdout]
             if self.stderr:
                 stderr = self._fileobj2output[self.stderr]

             self._save_input(input)

             if self._input:
                 input_view = memoryview(self._input)

             with _PopenSelector() as selector:
                 if self.stdin and input:
                     selector.register(self.stdin, selectors.EVENT_WRITE)
                 if self.stdout:
                     selector.register(self.stdout, selectors.EVENT_READ)
                 if self.stderr:
                     selector.register(self.stderr, selectors.EVENT_READ)

                 while selector.get_map():
                     timeout = self._remaining_time(endtime)
                     if timeout is not None and timeout < 0:
                         raise TimeoutExpired(self.args, orig_timeout)

                     ready = selector.select(timeout)
                     self._check_timeout(endtime, orig_timeout)

                     # XXX Rewrite these to use non-blocking I/O on the file
                     # objects; they are no longer using C stdio!

                     for key, events in ready:
                         if key.fileobj is self.stdin:
                             chunk = input_view[self._input_offset :
                                                self._input_offset + _PIPE_BUF]
                             try:
                                 self._input_offset += os.write(key.fd, chunk)
                             except BrokenPipeError:
                                 selector.unregister(key.fileobj)
                                 key.fileobj.close()
                             else:
                                 if self._input_offset >= len(self._input):
                                     selector.unregister(key.fileobj)
                                     key.fileobj.close()
                         elif key.fileobj in (self.stdout, self.stderr):
                             data = os.read(key.fd, 32768)
                             if not data:
                                 selector.unregister(key.fileobj)
                                 key.fileobj.close()
                             self._fileobj2output[key.fileobj].append(data)

             self.wait(timeout=self._remaining_time(endtime))

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

             # Translate newlines, if requested.
             # This also turns bytes into strings.
             if self.encoding or self.errors or self.universal_newlines:
                 if stdout is not None:
                     stdout = self._translate_newlines(stdout,
                                                       self.stdout.encoding,
                                                       self.stdout.errors)
                 if stderr is not None:
                     stderr = self._translate_newlines(stderr,
                                                       self.stderr.encoding,
                                                       self.stderr.errors)

             return (stdout, stderr)

         def _save_input(self, input):
             # This method is called from the _communicate_with_*() methods
             # so that if we time out while communicating, we can continue
             # sending input if we retry.
             if self.stdin and self._input is None:
                 self._input_offset = 0
                 self._input = input
                 if input is not None and (
                     self.encoding or self.errors or self.universal_newlines):
                     self._input = self._input.encode(self.stdin.encoding,
                                                      self.stdin.errors)

         def send_signal(self, sig):
             """Send a signal to the process."""
             # Skip signalling a process that we know has already died.
             if self.returncode is None:
                 os.kill(self.pid, sig)

         def terminate(self):
             """Terminate the process with SIGTERM
             """
             self.send_signal(signal.SIGTERM)

         def kill(self):
             """Kill the process with SIGKILL
             """
             self.send_signal(signal.SIGKILL)

python:subprocess