visit_list.sort(key=operator.itemgetter('datetime'))
    for key, group in groupby(visit_list, key=lambda x: myfn(x['datetime'])):
        for a in group:
            print({'datetime': a['datetime'], 'value': sum(int(d['value']) for d in group)})
def myfn(date):
    datetime_object = datetime.strptime(date, '%Y-%m-%dT%H:%M:%S')
    return datetime_object.hour