Read from the file descriptor and return the result as a string.

        The read_nonblocking method of :class:`SpawnBase` assumes that a call
        to os.read will not block (timeout parameter is ignored). This is not
        the case for POSIX file-like objects such as sockets and serial ports.

        Use :func:`select.select`, timeout is implemented conditionally for
        POSIX systems.

        :param int size: Read at most *size* bytes.
        :param int timeout: Wait timeout seconds for file descriptor to be
            ready to read. When -1 (default), use self.timeout. When 0, poll.
        :return: String containing the bytes read

def read_nonblocking(self, size=1, timeout=-1):
        """
        Read from the file descriptor and return the result as a string.

        The read_nonblocking method of :class:`SpawnBase` assumes that a call
        to os.read will not block (timeout parameter is ignored). This is not
        the case for POSIX file-like objects such as sockets and serial ports.

        Use :func:`select.select`, timeout is implemented conditionally for
        POSIX systems.

        :param int size: Read at most *size* bytes.
        :param int timeout: Wait timeout seconds for file descriptor to be
            ready to read. When -1 (default), use self.timeout. When 0, poll.
        :return: String containing the bytes read
        """
        if os.name == 'posix':
            if timeout == -1:
                timeout = self.timeout
            rlist = [self.child_fd]
            wlist = []
            xlist = []
            if self.use_poll:
                rlist = poll_ignore_interrupts(rlist, timeout)
            else:
                rlist, wlist, xlist = select_ignore_interrupts(
                    rlist, wlist, xlist, timeout
                )
            if self.child_fd not in rlist:
                raise TIMEOUT('Timeout exceeded.')
        return super(fdspawn, self).read_nonblocking(size)