This is used by the interact() method.

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

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

        while self.isalive():
            if self.use_poll:
                r = poll_ignore_interrupts([self.child_fd, self.STDIN_FILENO])
            else:
                r, w, e = select_ignore_interrupts(
                    [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)