Get exchange string for BaoStock

 Baostock 发送股票行情查询指令时需要带上交易所信息，以下是根据股票代码返回exchange字符串的函数，很多行情软件都是这个format，留存备用~

# Get stock exchange string
def GetExchange(stock_code_str):

    if (stock_code_str[:1] == '6'):    # 上交所-股票
        exchange_str = "sh"
    elif (stock_code_str[:1] == '5'):  # 上交所-ETF
        exchange_str = "sh"
    elif (stock_code_str[:1] == '3'):  # 深交所-股票
        exchange_str = "sz"
    elif (stock_code_str[:1] == '1'):  # 深交所-ETF
        exchange_str = "sz"
    elif (stock_code_str[:1] == '0'):  # 深交所-股票
        exchange_str = "sz"
    else:                              # reserved for 北交所
        exchange_str = ""
    # endif

    return exchange_str

# end def