range getting files from server

in a flask

def generate(taskname, ext='html'):
    #jobs=get_results_from_redis()
    #job = [i for i in jobs if i.name == taskname][0]
    try:
        logfile = open('/opt/deploy/var/log/tasks/%s.%s'%(taskname, ext), 'r')
    except:
        raise StopIteration

    yield ''.join(logfile.readlines())
    #while job.status != 'SUCCESS' and job.status != 'FAIL' and job.status != 'STOP' and logfile:
    try:
        line = logfile.readline()
        if line != '':
            if '\n' not in line:
                line = line + '\n'
            yield line
        else:
            #jobs=get_results_from_redis()
            #job = [i for i in jobs if i.name == taskname][0]
            #log.debug('no data while reading file for %s, state is %s,  sleeping', taskname, job.status)
            raise StopIteration
            time.sleep(2)
    except:
        app.logger.debug('exception occured!', exc_info=True)
        time.sleep(3)
        raise StopIteration


def send_file_partial(logname):
    path = '/opt/deploy/var/log/tasks/' + logname
    range_header = request.headers.get('Range', None)
    if not range_header: return send_file(path)

    size = os.path.getsize(path)
    byte1, byte2 = 0, None

    m = re.search('(\d+)-(\d*)', range_header)
    g = m.groups()

    if g[0]: byte1 = int(g[0])
    if g[1]: byte2 = int(g[1])

    length = size - byte1
    if byte2 is not None:
        length = byte2 - byte1

    data = None
    with open(path, 'rb') as f:
        f.seek(byte1)
        data = f.read(length)

    rv = Response(data,
        206,
        mimetype=mimetypes.guess_type(path)[0],
        direct_passthrough=True)
    rv.headers.add('Content-Range', 'bytes {0}-{1}/{2}'.format(byte1, byte1 + length - 1, size))

    return rv

@app.route('/log/')
@app.route('/log/<taskname>')
def stream_log(taskname=None):
    range_header = request.headers.get('Range')

    if taskname == None:
        return redirect(url_for('select'))
    if range_header is None:
        #log.debug('returning log without Range')
        return Response(stream_with_context(generate(taskname)), content_type='text/plain; charset=utf-8')
    else:
        #log.debug('returning Ranged log part')
        return send_file_partial(taskname + '.html')

in a page

<script type="text/javascript">
var xhr = new window.XMLHttpRequest();
var gotBytes = 0;
xhr.onreadystatechange = function () {
    if (xhr.readyState==4 && xhr.status>200 && xhr.status <400 && xhr.responseText !=''){
        var ins = document.getElementById('log');
        ins.innerHTML += xhr.responseText;
        gotBytes += xhr.responseText.length;
        if (xhr.responseText !="") 
            console.log('response text length is ', xhr.responseText.length, '. Overall length is now', ins.innerHTML.length, 'while requesting bytes starting from ', gotBytes);
        window.scrollTo(0,document.body.scrollHeight);
    }
    
};
function update () {
xhr.open("GET", "/log/{{taskname}}", true);
xhr.setRequestHeader('Content-Type', 'text/plain');
xhr.setRequestHeader('Tranfer-Encoding','chunked ');
xhr.setRequestHeader('Range', 'bytes='+gotBytes+'-');
xhr.send(null);
setTimeout(update, 3000);
}
update();
</script>