example with reading again

today = datetime.datetime.today()

for x in stocks:

    tws.reqHistoricalData(x)

    for n in periods:

        delta = datetime.timedelta(days=n)

        for _ in range(10): # repeat it only 10 times
            data = pd.DataFrame(reqHistoricalData(today-delta).strftime("%Y%m%d %H:%M:%S %Z"))

            if not data.empty:
                data.to_csv("filename{}@{}.csv".format(x, n))
                break # exit `for _` loop so it doesn't read again for the same period
            else:
                print("empty data for", x, n, '... reading again')
                tws.reqHistoricalData(x) # read again for the same period