def init(app_id, app_key=None, master_key=None):
    """初始化 LeanCloud 的 AppId / AppKey / MasterKey

    :type app_id: string_types
    :param app_id: 应用的 Application ID
    :type app_key: None or string_types
    :param app_key: 应用的 Application Key
    :type master_key: None or string_types
    :param master_key: 应用的 Master Key
    """
    if not app_key and not master_key:
        raise RuntimeError('app_key or master_key must be specified')
    global APP_ID, APP_KEY, MASTER_KEY
    APP_ID = app_id
    APP_KEY = app_key
    MASTER_KEY = master_key
