为什么我收到无效的语法错误？

Question

不确定为什么将名为“ CityPopYear2”的变量视为无效语法。我的目标是从我读取的csv文件中的数据中简单地找出一个城市在2年内的人口差异。因此，“ CityPopYear1”和“ CityPopYear2”仅用于减法公式中，代表2年各自的人口。不确定为什么将这样的简单方程式视为语法错误。

def CityPopChange():
    f = open("CityPop.csv","r")
    header = f.readline()
    header = header.strip().split(",")
    CityPop = {}
    for line in f:
        record = line.strip().split(",")
        CityPop[record[4]] = record
    CityName = raw_input("What city would you like to view?: ")
    CityYear1 = raw_input("Please enter the earliest year you would like studied: ")
    CityYear2 = raw_input("Please enter the most recent year you would like studied: ")
    i = 0
    while i < len(header):
        if header[i] == CityYear1:
            break
        i = i +1
    j = 0
    while j < len(header):
        if header[i] == CityYear2:
            break
        j = j + 1
    CityPopYear1 = float((CityPop[CityYear1][i])
    CityPopYear2 = float((CityPop[CityYear2][j])

    PopChange = CityPopYear2 - CityPopYear1

    print "The difference in population in", CityName, "between", CityYear1, "and", CityYear2, "is", PopChange, "."

CityPopChange()