pexpect.pty_spawn — Pexpect 4.1.0 documentation

# Source code for pexpect.pty_spawn

    import os
    import sys
    import time
    import select
    import pty
    import tty
    import errno
    import signal
    from contextlib import contextmanager

    import ptyprocess
    from ptyprocess.ptyprocess import use_native_pty_fork

    from .exceptions import ExceptionPexpect, EOF, TIMEOUT
    from .spawnbase import SpawnBase
    from .utils import which, split_command_line

    @contextmanager
    def _wrap_ptyprocess_err():
        """Turn ptyprocess errors into our own ExceptionPexpect errors"""
        try:
            yield
        except ptyprocess.PtyProcessError as e:
            raise ExceptionPexpect(*e.args)

    PY3 = (sys.version_info[0] >= 3)

    class spawn(SpawnBase):
        '''This is the main class interface for Pexpect. Use this class to start
        and control child applications. '''

        # This is purely informational now - changing it has no effect
        use_native_pty_fork = use_native_pty_fork

        def __init__(self, command, args=[], timeout=30, maxread=2000,
                     searchwindowsize=None, logfile=None, cwd=None, env=None,
                     ignore_sighup=False, echo=True, preexec_fn=None,
                     encoding=None, codec_errors='strict', dimensions=None):
            '''This is the constructor. The command parameter may be a string that
            includes a command and any arguments to the command. For example::

                child = pexpect.spawn('/usr/bin/ftp')
                child = pexpect.spawn('/usr/bin/ssh user@example.com')
                child = pexpect.spawn('ls -latr /tmp')

            You may also construct it with a list of arguments like so::

                child = pexpect.spawn('/usr/bin/ftp', [])
                child = pexpect.spawn('/usr/bin/ssh', ['user@example.com'])
                child = pexpect.spawn('ls', ['-latr', '/tmp'])

            After this the child application will be created and will be ready to
            talk to. For normal use, see expect() and send() and sendline().

            Remember that Pexpect does NOT interpret shell meta characters such as
            redirect, pipe, or wild cards (``>``, ``|``, or ``*``). This is a
            common mistake.  If you want to run a command and pipe it through
            another command then you must also start a shell. For example::

                child = pexpect.spawn('/bin/bash -c "ls -l | grep LOG > logs.txt"')
                child.expect(pexpect.EOF)

            The second form of spawn (where you pass a list of arguments) is useful
            in situations where you wish to spawn a command and pass it its own
            argument list. This can make syntax more clear. For example, the
            following is equivalent to the previous example::

                shell_cmd = 'ls -l | grep LOG > logs.txt'
                child = pexpect.spawn('/bin/bash', ['-c', shell_cmd])
                child.expect(pexpect.EOF)

            The maxread attribute sets the read buffer size. This is maximum number
            of bytes that Pexpect will try to read from a TTY at one time. Setting
            the maxread size to 1 will turn off buffering. Setting the maxread
            value higher may help performance in cases where large amounts of
            output are read back from the child. This feature is useful in
            conjunction with searchwindowsize.

            When the keyword argument *searchwindowsize* is None (default), the
            full buffer is searched at each iteration of receiving incoming data.
            The default number of bytes scanned at each iteration is very large
            and may be reduced to collaterally reduce search cost.  After
            :meth:`~.expect` returns, the full buffer attribute remains up to
            size *maxread* irrespective of *searchwindowsize* value.

            When the keyword argument ``timeout`` is specified as a number,
            (default: *30*), then :class:`TIMEOUT` will be raised after the value
            specified has elapsed, in seconds, for any of the :meth:`~.expect`
            family of method calls.  When None, TIMEOUT will not be raised, and
            :meth:`~.expect` may block indefinitely until match.

            The logfile member turns on or off logging. All input and output will
            be copied to the given file object. Set logfile to None to stop
            logging. This is the default. Set logfile to sys.stdout to echo
            everything to standard output. The logfile is flushed after each write.

            Example log input and output to a file::

                child = pexpect.spawn('some_command')
                fout = open('mylog.txt','wb')
                child.logfile = fout

            Example log to stdout::

                # In Python 2:
                child = pexpect.spawn('some_command')
                child.logfile = sys.stdout

                # In Python 3, spawnu should be used to give str to stdout:
                child = pexpect.spawnu('some_command')
                child.logfile = sys.stdout

            The logfile_read and logfile_send members can be used to separately log
            the input from the child and output sent to the child. Sometimes you
            don't want to see everything you write to the child. You only want to
            log what the child sends back. For example::

                child = pexpect.spawn('some_command')
                child.logfile_read = sys.stdout

            You will need to pass an encoding to spawn in the above code if you are
            using Python 3.

            To separately log output sent to the child use logfile_send::

                child.logfile_send = fout

            If ``ignore_sighup`` is True, the child process will ignore SIGHUP
            signals. The default is False from Pexpect 4.0, meaning that SIGHUP
            will be handled normally by the child.

            The delaybeforesend helps overcome a weird behavior that many users
            were experiencing. The typical problem was that a user would expect() a
            "Password:" prompt and then immediately call sendline() to send the
            password. The user would then see that their password was echoed back
            to them. Passwords don't normally echo. The problem is caused by the
            fact that most applications print out the "Password" prompt and then
            turn off stdin echo, but if you send your password before the
            application turned off echo, then you get your password echoed.
            Normally this wouldn't be a problem when interacting with a human at a
            real keyboard. If you introduce a slight delay just before writing then
            this seems to clear up the problem. This was such a common problem for
            many users that I decided that the default pexpect behavior should be
            to sleep just before writing to the child application. 1/20th of a
            second (50 ms) seems to be enough to clear up the problem. You can set
            delaybeforesend to None to return to the old behavior.

            Note that spawn is clever about finding commands on your path.
            It uses the same logic that "which" uses to find executables.

            If you wish to get the exit status of the child you must call the
            close() method. The exit or signal status of the child will be stored
            in self.exitstatus or self.signalstatus. If the child exited normally
            then exitstatus will store the exit return code and signalstatus will
            be None. If the child was terminated abnormally with a signal then
            signalstatus will store the signal value and exitstatus will be None::

                child = pexpect.spawn('some_command')
                child.close()
                print(child.exitstatus, child.signalstatus)

            If you need more detail you can also read the self.status member which
            stores the status returned by os.waitpid. You can interpret this using
            os.WIFEXITED/os.WEXITSTATUS or os.WIFSIGNALED/os.TERMSIG.

            The echo attribute may be set to False to disable echoing of input.
            As a pseudo-terminal, all input echoed by the "keyboard" (send()
            or sendline()) will be repeated to output.  For many cases, it is
            not desirable to have echo enabled, and it may be later disabled
            using setecho(False) followed by waitnoecho().  However, for some
            platforms such as Solaris, this is not possible, and should be
            disabled immediately on spawn.

            If preexec_fn is given, it will be called in the child process before
            launching the given command. This is useful to e.g. reset inherited
            signal handlers.

            The dimensions attribute specifies the size of the pseudo-terminal as
            seen by the subprocess, and is specified as a two-entry tuple (rows,
            columns). If this is unspecified, the defaults in ptyprocess will apply.
            '''
            super(spawn, self).__init__(timeout=timeout, maxread=maxread, searchwindowsize=searchwindowsize,
                                        logfile=logfile, encoding=encoding, codec_errors=codec_errors)
            self.STDIN_FILENO = pty.STDIN_FILENO
            self.STDOUT_FILENO = pty.STDOUT_FILENO
            self.STDERR_FILENO = pty.STDERR_FILENO
            self.cwd = cwd
            self.env = env
            self.echo = echo
            self.ignore_sighup = ignore_sighup
            self.__irix_hack = sys.platform.lower().startswith('irix')
            if command is None:
                self.command = None
                self.args = None
                self.name = '<pexpect factory incomplete>'
            else:
                self._spawn(command, args, preexec_fn, dimensions)

        def __str__(self):
            '''This returns a human-readable string that represents the state of
            the object. '''

            s = []
            s.append(repr(self))
            s.append('command: ' + str(self.command))
            s.append('args: %r' % (self.args,))
            s.append('buffer (last 100 chars): %r' % (
                    self.buffer[-100:] if self.buffer else self.buffer,))
            s.append('before (last 100 chars): %r' % (
                    self.before[-100:] if self.before else self.before,))
            s.append('after: %r' % (self.after,))
            s.append('match: %r' % (self.match,))
            s.append('match_index: ' + str(self.match_index))
            s.append('exitstatus: ' + str(self.exitstatus))
            if hasattr(self, 'ptyproc'):
                s.append('flag_eof: ' + str(self.flag_eof))
            s.append('pid: ' + str(self.pid))
            s.append('child_fd: ' + str(self.child_fd))
            s.append('closed: ' + str(self.closed))
            s.append('timeout: ' + str(self.timeout))
            s.append('delimiter: ' + str(self.delimiter))
            s.append('logfile: ' + str(self.logfile))
            s.append('logfile_read: ' + str(self.logfile_read))
            s.append('logfile_send: ' + str(self.logfile_send))
            s.append('maxread: ' + str(self.maxread))
            s.append('ignorecase: ' + str(self.ignorecase))
            s.append('searchwindowsize: ' + str(self.searchwindowsize))
            s.append('delaybeforesend: ' + str(self.delaybeforesend))
            s.append('delayafterclose: ' + str(self.delayafterclose))
            s.append('delayafterterminate: ' + str(self.delayafterterminate))
            return '\n'.join(s)

        def _spawn(self, command, args=[], preexec_fn=None, dimensions=None):
            '''This starts the given command in a child process. This does all the
            fork/exec type of stuff for a pty. This is called by __init__. If args
            is empty then command will be parsed (split on spaces) and args will be
            set to parsed arguments. '''

            # The pid and child_fd of this object get set by this method.
            # Note that it is difficult for this method to fail.
            # You cannot detect if the child process cannot start.
            # So the only way you can tell if the child process started
            # or not is to try to read from the file descriptor. If you get
            # EOF immediately then it means that the child is already dead.
            # That may not necessarily be bad because you may have spawned a child
            # that performs some task; creates no stdout output; and then dies.

            # If command is an int type then it may represent a file descriptor.
            if isinstance(command, type(0)):
                raise ExceptionPexpect('Command is an int type. ' +
                        'If this is a file descriptor then maybe you want to ' +
                        'use fdpexpect.fdspawn which takes an existing ' +
                        'file descriptor instead of a command string.')

            if not isinstance(args, type([])):
                raise TypeError('The argument, args, must be a list.')

            if args == []:
                self.args = split_command_line(command)
                self.command = self.args[0]
            else:
                # Make a shallow copy of the args list.
                self.args = args[:]
                self.args.insert(0, command)
                self.command = command

            command_with_path = which(self.command)
            if command_with_path is None:
                raise ExceptionPexpect('The command was not found or was not ' +
                        'executable: %s.' % self.command)
            self.command = command_with_path
            self.args[0] = self.command

            self.name = '<' + ' '.join(self.args) + '>'

            assert self.pid is None, 'The pid member must be None.'
            assert self.command is not None, 'The command member must not be None.'

            kwargs = {'echo': self.echo, 'preexec_fn': preexec_fn}
            if self.ignore_sighup:
                def preexec_wrapper():
                    "Set SIGHUP to be ignored, then call the real preexec_fn"
                    signal.signal(signal.SIGHUP, signal.SIG_IGN)
                    if preexec_fn is not None:
                        preexec_fn()
                kwargs['preexec_fn'] = preexec_wrapper

            if dimensions is not None:
                kwargs['dimensions'] = dimensions

            if self.encoding is not None:
                # Encode command line using the specified encoding
                self.args = [a if isinstance(a, bytes) else a.encode(self.encoding)
                             for a in self.args]

            self.ptyproc = self._spawnpty(self.args, env=self.env,
                                         cwd=self.cwd, **kwargs)

            self.pid = self.ptyproc.pid
            self.child_fd = self.ptyproc.fd

            self.terminated = False
            self.closed = False

        def _spawnpty(self, args, **kwargs):
            '''Spawn a pty and return an instance of PtyProcess.'''
            return ptyprocess.PtyProcess.spawn(args, **kwargs)

        def close(self, force=True):
            '''This closes the connection with the child application. Note that
            calling close() more than once is valid. This emulates standard Python
            behavior with files. Set force to True if you want to make sure that
            the child is terminated (SIGKILL is sent if the child ignores SIGHUP
            and SIGINT). '''

            self.flush()
            self.ptyproc.close(force=force)
            self.isalive()  # Update exit status from ptyproc
            self.child_fd = -1

        def isatty(self):
            '''This returns True if the file descriptor is open and connected to a
            tty(-like) device, else False.

            On SVR4-style platforms implementing streams, such as SunOS and HP-UX,
            the child pty may not appear as a terminal device.  This means
            methods such as setecho(), setwinsize(), getwinsize() may raise an
            IOError. '''

            return os.isatty(self.child_fd)

        def waitnoecho(self, timeout=-1):
            '''This waits until the terminal ECHO flag is set False. This returns
            True if the echo mode is off. This returns False if the ECHO flag was
            not set False before the timeout. This can be used to detect when the
            child is waiting for a password. Usually a child application will turn
            off echo mode when it is waiting for the user to enter a password. For
            example, instead of expecting the "password:" prompt you can wait for
            the child to set ECHO off::

                p = pexpect.spawn('ssh user@example.com')
                p.waitnoecho()
                p.sendline(mypassword)

            If timeout==-1 then this method will use the value in self.timeout.
            If timeout==None then this method to block until ECHO flag is False.
            '''

            if timeout == -1:
                timeout = self.timeout
            if timeout is not None:
                end_time = time.time() + timeout
            while True:
                if not self.getecho():
                    return True
                if timeout < 0 and timeout is not None:
                    return False
                if timeout is not None:
                    timeout = end_time - time.time()
                time.sleep(0.1)

        def getecho(self):
            '''This returns the terminal echo mode. This returns True if echo is
            on or False if echo is off. Child applications that are expecting you
            to enter a password often set ECHO False. See waitnoecho().

            Not supported on platforms where ``isatty()`` returns False.  '''
            return self.ptyproc.getecho()

        def setecho(self, state):
            '''This sets the terminal echo mode on or off. Note that anything the
            child sent before the echo will be lost, so you should be sure that
            your input buffer is empty before you call setecho(). For example, the
            following will work as expected::

                p = pexpect.spawn('cat') # Echo is on by default.
                p.sendline('1234') # We expect see this twice from the child...
                p.expect(['1234']) # ... once from the tty echo...
                p.expect(['1234']) # ... and again from cat itself.
                p.setecho(False) # Turn off tty echo
                p.sendline('abcd') # We will set this only once (echoed by cat).
                p.sendline('wxyz') # We will set this only once (echoed by cat)
                p.expect(['abcd'])
                p.expect(['wxyz'])

            The following WILL NOT WORK because the lines sent before the setecho
            will be lost::

                p = pexpect.spawn('cat')
                p.sendline('1234')
                p.setecho(False) # Turn off tty echo
                p.sendline('abcd') # We will set this only once (echoed by cat).
                p.sendline('wxyz') # We will set this only once (echoed by cat)
                p.expect(['1234'])
                p.expect(['1234'])
                p.expect(['abcd'])
                p.expect(['wxyz'])

            Not supported on platforms where ``isatty()`` returns False.
            '''
            return self.ptyproc.setecho(state)

            self.echo = state

        def read_nonblocking(self, size=1, timeout=-1):
            '''This reads at most size characters from the child application. It
            includes a timeout. If the read does not complete within the timeout
            period then a TIMEOUT exception is raised. If the end of file is read
            then an EOF exception will be raised.  If a logfile is specified, a
            copy is written to that log.

            If timeout is None then the read may block indefinitely.
            If timeout is -1 then the self.timeout value is used. If timeout is 0
            then the child is polled and if there is no data immediately ready
            then this will raise a TIMEOUT exception.

            The timeout refers only to the amount of time to read at least one
            character. This is not affected by the 'size' parameter, so if you call
            read_nonblocking(size=100, timeout=30) and only one character is
            available right away then one character will be returned immediately.
            It will not wait for 30 seconds for another 99 characters to come in.

            This is a wrapper around os.read(). It uses select.select() to
            implement the timeout. '''

            if self.closed:
                raise ValueError('I/O operation on closed file.')

            if timeout == -1:
                timeout = self.timeout

            # Note that some systems such as Solaris do not give an EOF when
            # the child dies. In fact, you can still try to read
            # from the child_fd -- it will block forever or until TIMEOUT.
            # For this case, I test isalive() before doing any reading.
            # If isalive() is false, then I pretend that this is the same as EOF.
            if not self.isalive():
                # timeout of 0 means "poll"
                r, w, e = self.__select([self.child_fd], [], [], 0)
                if not r:
                    self.flag_eof = True
                    raise EOF('End Of File (EOF). Braindead platform.')
            elif self.__irix_hack:
                # Irix takes a long time before it realizes a child was terminated.
                # FIXME So does this mean Irix systems are forced to always have
                # FIXME a 2 second delay when calling read_nonblocking? That sucks.
                r, w, e = self.__select([self.child_fd], [], [], 2)
                if not r and not self.isalive():
                    self.flag_eof = True
                    raise EOF('End Of File (EOF). Slow platform.')

            r, w, e = self.__select([self.child_fd], [], [], timeout)

            if not r:
                if not self.isalive():
                    # Some platforms, such as Irix, will claim that their
                    # processes are alive; timeout on the select; and
                    # then finally admit that they are not alive.
                    self.flag_eof = True
                    raise EOF('End of File (EOF). Very slow platform.')
                else:
                    raise TIMEOUT('Timeout exceeded.')

            if self.child_fd in r:
                return super(spawn, self).read_nonblocking(size)

            raise ExceptionPexpect('Reached an unexpected state.')  # pragma: no cover

        def write(self, s):
            '''This is similar to send() except that there is no return value.
            '''

            self.send(s)

        def writelines(self, sequence):
            '''This calls write() for each element in the sequence. The sequence
            can be any iterable object producing strings, typically a list of
            strings. This does not add line separators. There is no return value.
            '''

            for s in sequence:
                self.write(s)

        def send(self, s):
            '''Sends string ``s`` to the child process, returning the number of
            bytes written. If a logfile is specified, a copy is written to that
            log.

            The default terminal input mode is canonical processing unless set
            otherwise by the child process. This allows backspace and other line
            processing to be performed prior to transmitting to the receiving
            program. As this is buffered, there is a limited size of such buffer.

            On Linux systems, this is 4096 (defined by N_TTY_BUF_SIZE). All
            other systems honor the POSIX.1 definition PC_MAX_CANON -- 1024
            on OSX, 256 on OpenSolaris, and 1920 on FreeBSD.

            This value may be discovered using fpathconf(3)::

                >>> from os import fpathconf
                >>> print(fpathconf(0, 'PC_MAX_CANON'))
                256

            On such a system, only 256 bytes may be received per line. Any
            subsequent bytes received will be discarded. BEL (``'\a'``) is then
            sent to output if IMAXBEL (termios.h) is set by the tty driver.
            This is usually enabled by default.  Linux does not honor this as
            an option -- it behaves as though it is always set on.

            Canonical input processing may be disabled altogether by executing
            a shell, then stty(1), before executing the final program::

                >>> bash = pexpect.spawn('/bin/bash', echo=False)
                >>> bash.sendline('stty -icanon')
                >>> bash.sendline('base64')
                >>> bash.sendline('x' * 5000)
            '''

            if self.delaybeforesend is not None:
                time.sleep(self.delaybeforesend)

            s = self._coerce_send_string(s)
            self._log(s, 'send')

            b = self._encoder.encode(s, final=False)
            return os.write(self.child_fd, b)

        def sendline(self, s=''):
            '''Wraps send(), sending string ``s`` to child process, with
            ``os.linesep`` automatically appended. Returns number of bytes
            written.  Only a limited number of bytes may be sent for each
            line in the default terminal mode, see docstring of :meth:`send`.
            '''
            s = self._coerce_send_string(s)
            return self.send(s + self.linesep)

        def _log_control(self, s):
            """Write control characters to the appropriate log files"""
            if self.encoding is not None:
                s = s.decode(self.encoding, 'replace')
            self._log(s, 'send')

        def sendcontrol(self, char):
            '''Helper method that wraps send() with mnemonic access for sending control
            character to the child (such as Ctrl-C or Ctrl-D).  For example, to send
            Ctrl-G (ASCII 7, bell, '\a')::

                child.sendcontrol('g')

            See also, sendintr() and sendeof().
            '''
            n, byte = self.ptyproc.sendcontrol(char)
            self._log_control(byte)
            return n

        def sendeof(self):
            '''This sends an EOF to the child. This sends a character which causes
            the pending parent output buffer to be sent to the waiting child
            program without waiting for end-of-line. If it is the first character
            of the line, the read() in the user program returns 0, which signifies
            end-of-file. This means to work as expected a sendeof() has to be
            called at the beginning of a line. This method does not send a newline.
            It is the responsibility of the caller to ensure the eof is sent at the
            beginning of a line. '''

            n, byte = self.ptyproc.sendeof()
            self._log_control(byte)

        def sendintr(self):
            '''This sends a SIGINT to the child. It does not require
            the SIGINT to be the first character on a line. '''

            n, byte = self.ptyproc.sendintr()
            self._log_control(byte)

        @property
        def flag_eof(self):
            return self.ptyproc.flag_eof

        @flag_eof.setter
        def flag_eof(self, value):
            self.ptyproc.flag_eof = value

        def eof(self):
            '''This returns True if the EOF exception was ever raised.
            '''
            return self.flag_eof

        def terminate(self, force=False):
            '''This forces a child process to terminate. It starts nicely with
            SIGHUP and SIGINT. If "force" is True then moves onto SIGKILL. This
            returns True if the child was terminated. This returns False if the
            child could not be terminated. '''

            if not self.isalive():
                return True
            try:
                self.kill(signal.SIGHUP)
                time.sleep(self.delayafterterminate)
                if not self.isalive():
                    return True
                self.kill(signal.SIGCONT)
                time.sleep(self.delayafterterminate)
                if not self.isalive():
                    return True
                self.kill(signal.SIGINT)
                time.sleep(self.delayafterterminate)
                if not self.isalive():
                    return True
                if force:
                    self.kill(signal.SIGKILL)
                    time.sleep(self.delayafterterminate)
                    if not self.isalive():
                        return True
                    else:
                        return False
                return False
            except OSError:
                # I think there are kernel timing issues that sometimes cause
                # this to happen. I think isalive() reports True, but the
                # process is dead to the kernel.
                # Make one last attempt to see if the kernel is up to date.
                time.sleep(self.delayafterterminate)
                if not self.isalive():
                    return True
                else:
                    return False

        def wait(self):
            '''This waits until the child exits. This is a blocking call. This will
            not read any data from the child, so this will block forever if the
            child has unread output and has terminated. In other words, the child
            may have printed output then called exit(), but, the child is
            technically still alive until its output is read by the parent.

            This method is non-blocking if :meth:`wait` has already been called
            previously or :meth:`isalive` method returns False.  It simply returns
            the previously determined exit status.
            '''

            ptyproc = self.ptyproc
            with _wrap_ptyprocess_err():
                # exception may occur if "Is some other process attempting
                # "job control with our child pid?"
                exitstatus = ptyproc.wait()
            self.status = ptyproc.status
            self.exitstatus = ptyproc.exitstatus
            self.signalstatus = ptyproc.signalstatus
            self.terminated = True

            return exitstatus

        def isalive(self):
            '''This tests if the child process is running or not. This is
            non-blocking. If the child was terminated then this will read the
            exitstatus or signalstatus of the child. This returns True if the child
            process appears to be running or False if not. It can take literally
            SECONDS for Solaris to return the right status. '''

            ptyproc = self.ptyproc
            with _wrap_ptyprocess_err():
                alive = ptyproc.isalive()

            if not alive:
                self.status = ptyproc.status
                self.exitstatus = ptyproc.exitstatus
                self.signalstatus = ptyproc.signalstatus
                self.terminated = True

            return alive

        def kill(self, sig):

            '''This sends the given signal to the child application. In keeping
            with UNIX tradition it has a misleading name. It does not necessarily
            kill the child unless you send the right signal. '''

            # Same as os.kill, but the pid is given for you.
            if self.isalive():
                os.kill(self.pid, sig)

        def getwinsize(self):
            '''This returns the terminal window size of the child tty. The return
            value is a tuple of (rows, cols). '''
            return self.ptyproc.getwinsize()

        def setwinsize(self, rows, cols):
            '''This sets the terminal window size of the child tty. This will cause
            a SIGWINCH signal to be sent to the child. This does not change the
            physical window size. It changes the size reported to TTY-aware
            applications like vi or curses -- applications that respond to the
            SIGWINCH signal. '''
            return self.ptyproc.setwinsize(rows, cols)

        def interact(self, escape_character=chr(29),
                input_filter=None, output_filter=None):

            '''This gives control of the child process to the interactive user (the
            human at the keyboard). Keystrokes are sent to the child process, and
            the stdout and stderr output of the child process is printed. This
            simply echos the child stdout and child stderr to the real stdout and
            it echos the real stdin to the child stdin. When the user types the
            escape_character this method will return None. The escape_character
            will not be transmitted.  The default for escape_character is
            entered as ``Ctrl - ]``, the very same as BSD telnet. To prevent
            escaping, escape_character may be set to None.

            If a logfile is specified, then the data sent and received from the
            child process in interact mode is duplicated to the given log.

            You may pass in optional input and output filter functions. These
            functions should take a string and return a string. The output_filter
            will be passed all the output from the child process. The input_filter
            will be passed all the keyboard input from the user. The input_filter
            is run BEFORE the check for the escape_character.

            Note that if you change the window size of the parent the SIGWINCH
            signal will not be passed through to the child. If you want the child
            window size to change when the parent's window size changes then do
            something like the following example::

                import pexpect, struct, fcntl, termios, signal, sys
                def sigwinch_passthrough (sig, data):
                    s = struct.pack("HHHH", 0, 0, 0, 0)
                    a = struct.unpack('hhhh', fcntl.ioctl(sys.stdout.fileno(),
                        termios.TIOCGWINSZ , s))
                    global p
                    p.setwinsize(a[0],a[1])
                # Note this 'p' global and used in sigwinch_passthrough.
                p = pexpect.spawn('/bin/bash')
                signal.signal(signal.SIGWINCH, sigwinch_passthrough)
                p.interact()
            '''

            # Flush the buffer.
            self.write_to_stdout(self.buffer)
            self.stdout.flush()
            self.buffer = self.string_type()
            mode = tty.tcgetattr(self.STDIN_FILENO)
            tty.setraw(self.STDIN_FILENO)
            if escape_character is not None and PY3:
                escape_character = escape_character.encode('latin-1')
            try:
                self.__interact_copy(escape_character, input_filter, output_filter)
            finally:
                tty.tcsetattr(self.STDIN_FILENO, tty.TCSAFLUSH, mode)

        def __interact_writen(self, fd, data):
            '''This is used by the interact() method.
            '''

            while data != b'' and self.isalive():
                n = os.write(fd, data)
                data = data[n:]

        def __interact_read(self, fd):
            '''This is used by the interact() method.
            '''

            return os.read(fd, 1000)

        def __interact_copy(self, escape_character=None,
                input_filter=None, output_filter=None):

            '''This is used by the interact() method.
            '''

            while self.isalive():
                r, w, e = self.__select([self.child_fd, self.STDIN_FILENO], [], [])
                if self.child_fd in r:
                    try:
                        data = self.__interact_read(self.child_fd)
                    except OSError as err:
                        if err.args[0] == errno.EIO:
                            # Linux-style EOF
                            break
                        raise
                    if data == b'':
                        # BSD-style EOF
                        break
                    if output_filter:
                        data = output_filter(data)
                    self._log(data, 'read')
                    os.write(self.STDOUT_FILENO, data)
                if self.STDIN_FILENO in r:
                    data = self.__interact_read(self.STDIN_FILENO)
                    if input_filter:
                        data = input_filter(data)
                    i = -1
                    if escape_character is not None:
                        i = data.rfind(escape_character)
                    if i != -1:
                        data = data[:i]
                        if data:
                            self._log(data, 'send')
                        self.__interact_writen(self.child_fd, data)
                        break
                    self._log(data, 'send')
                    self.__interact_writen(self.child_fd, data)

        def __select(self, iwtd, owtd, ewtd, timeout=None):

            '''This is a wrapper around select.select() that ignores signals. If
            select.select raises a select.error exception and errno is an EINTR
            error then it is ignored. Mainly this is used to ignore sigwinch
            (terminal resize). '''

            # if select() is interrupted by a signal (errno==EINTR) then
            # we loop back and enter the select() again.
            if timeout is not None:
                end_time = time.time() + timeout
            while True:
                try:
                    return select.select(iwtd, owtd, ewtd, timeout)
                except select.error:
                    err = sys.exc_info()[1]
                    if err.args[0] == errno.EINTR:
                        # if we loop back we have to subtract the
                        # amount of time we already waited.
                        if timeout is not None:
                            timeout = end_time - time.time()
                            if timeout < 0:
                                return([], [], [])
                    else:
                        # something else caused the select.error, so
                        # this actually is an exception.
                        raise

    def spawnu(*args, **kwargs):
        """Deprecated: pass encoding to spawn() instead."""
        kwargs.setdefault('encoding', 'utf-8')
        return spawn(*args, **kwargs)