PythonXLWT简单wirting to sheet示例

有人能解释为什么我会出现以下错误：

File "C:\Users\Saul Ramirez\Documents\Python Scripts\DQforecast.py", line 156, in xlsOutput
forecastSheet.write(index, i, 'asdf')

TypeError: object of type 'int' has no len()    

对于此功能

def xlsOutput(forecasts):
    os.chdir('C:\\Users\\Saul Ramirez\\Desktop')
    forecastWB = xlwt.Workbook()
    forecastWB_name = 'Forecasts' + str(datetime.date.today()) + '.xls'
    forecastSheet = forecastWB.add_sheet('Forecasst')
    i=0
    for rollRate in forecasts:
        for index in rollRate:
            forecastSheet.write(index, i, 'asdf')
        i=i+1

forecastWB.save(forecastWB_name)

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显示完整的回溯-关于xlwt的哪个部分试图调用len的信息是必要的。找出它。。。愚蠢的问题对不起！