pyserial write() method.

pyserial.write.py

class Serial(object):
    ....
    def write(self, data):
        """Output the given string over the serial port."""
        if not self._isOpen: raise portNotOpenError
        t = len(data)
        d = data
        if self._writeTimeout is not None and self._writeTimeout > 0:
            timeout = time.time() + self._writeTimeout
        else:
            timeout = None
        while t > 0:
            try:
                n = os.write(self.fd, d)
                if timeout:
                    # when timeout is set, use select to wait for being ready
                    # with the time left as timeout
                    timeleft = timeout - time.time()
                    if timeleft < 0:
                        raise writeTimeoutError
                    _, ready, _ = select.select([], [self.fd], [], timeleft)
                    if not ready:
                        raise writeTimeoutError
                d = d[n:]
                t = t - n
            except OSError, v:
                if v.errno != errno.EAGAIN:
                    raise SerialException('write failed: %s' % (v,))
        return len(data)