以下語句中，QBDownloaderDayPrice 繼承自 QBDownloader，執(zhí)行時報錯AttributeError: 'QBDownloaderDayPrice' object has no attribute '_QBDownloader__flag_encoding'。請教如何解決？

import time
import datetime
import tushare as ts
import pandas as pd

class QBDownloader(object):

def __init__(self):
    self.__filename = ''
    self.__df_data = null
    self.__flag_encoding = 1

def saveto_csv(self):
    if self.__flag_encoding == 1:
        self.__df_data.to_csv(self.__filename, encoding='GBK')
    else:
        self.__df_data.to_csv(self.__filename)

class QBDownloaderDayPrice(QBDownloader):

def __init__(self):
    self.__filename = 'QBdata\\DayPrice\\dp' + time.strftime("%Y%m%d", time.localtime()) + '.csv'
    self.__df_data = self.get_df_data()
    self.__flag_encoding = 1

def get_df_data(self):
    #獲取當天所有股票日價格，返回dataframe
    df_day_price = ts.get_today_all()
    return df_day_price

if name == '__main__':

obj_dl = QBDownloaderDayPrice()
obj_dl.saveto_csv()
print u'存儲成功'