def getUsernames(tablename):
  try:
    db = MySQLdb.connect(host="localhost", user="root", password="root", database="contacts", port=5432)
    cur = db.cursor()
    cur.execute("select user from %s"%(tablename))
    usernames = list(map(lambda x: x[0], cur.fetchall()))
    db.close()
    return json.dumps(usernames,ensure_ascii=False,indent=2,sort_keys=True)
  except MySQLdb.Error as error:
    print('ERROR in getUsernames\n', error)
    return None