为什么我的python中的for循环不会迭代到列表末尾？

Question

我是python的新手，我正在用python编写一个程序，该程序将获取日期和汽油价格的列表，并返回一年中每个月的平均值。我现在拥有的代码在第二个月到上个月停止，我不知道为什么。我知道可能有一种更有效的编写方法，因此任何有关此的建议也将有所帮助。任何事情都会有所帮助！谢谢。

def readGasPrices(fileName):
    gasFile = open(fileName, 'r')
    gasFileList = gasFile.readlines()
    priceList = []
    for line in gasFileList:
        priceList.append(line[11:].rstrip('\n'))
    gasFile.close
    return priceList

def readGasDates(fileName):
    gasFile = open(fileName, 'r')
    gasFileList = gasFile.readlines()
    dateList = []
    for line in gasFileList:
        dateList.append(line[0:10])
    gasFile.close
    return dateList

def average_price_per_month(dates,prices):
    month = ''
    year = ''
    counter = 0
    accumulator = 0


    for index in range(len(dates)):
        if year == '':
            year = dates[index][6:10]
            currentYear = year

        elif month == '':
            month = dates[index][0:2]
            print('Gas price averages for', currentYear)
            print('Month'.center(22) + 'Average'.center(12))
            print('====='.center(22) + '======='.center(12))

        elif dates[index][0:2] == month and dates[index][6:10] == currentYear:
            accumulator += float(prices[index])
            counter += 1

        elif year != currentYear:
            currentYear = year
            print()
            print('Gas price averages for', currentYear)
            print('Month'.center(22) + 'Average'.center(12))
            print('====='.center(22) + '======='.center(12))

        else:
            if month == '01':
                month = 'January'
            elif month == '02':
                month = 'February'
            elif month == '03':
                month = 'March'
            elif month == '04':
                month = 'April'
            elif month == '05':
                month = 'May'
            elif month == '06':
                month = 'June'
            elif month == '07':
                month = 'July'
            elif month == '08':
                month = 'August'
            elif month == '09':
                month = 'September'
            elif month == '10':
                month = 'October'
            elif month == '11':
                month = 'November'
            elif month == '12':
                month = 'December'

            average = accumulator / counter
            print(month.ljust(10).center(25) + \
                      format(average,'.2f').center(0))

            counter = 1
            accumulator = float(prices[index])
            year = dates[index][6:10]
            month = dates[index][0:2]


def main():
    fileName = input('Enter the file name: ')
    gasPriceList = readGasPrices(fileName)
    dateList = readGasDates(fileName)
    average_price_per_month(dateList, gasPriceList)

main()

Example of what the file looks like