即使没有必要在python中初始化变量，我的教授仍然希望我们将其用于实践。我编写了程序，但运行良好，但是尝试初始化一些变量后，尝试运行该程序时收到错误消息。这是我程序的第一部分：

def main():

    grade_1, grade_2, grade_3, average = 0.0
    year = 0

    fName, lName, ID, converted_ID = ""
    infile = open("studentinfo.txt", "r")
    data = infile.read()
    fName, lName, ID, year = data.split(",")
    year = int(year)

    # Prompt the user for three test scores

    grades = eval(input("Enter the three test scores separated by a comma: "))

    # Create a username

    uName = (lName[:4] + fName[:2] + str(year)).lower()
    converted_id = ID[:3] + "-" + ID[3:5] + "-" + ID[5:]
    grade_1, grade_2, grade_3 = grades

错误信息：

grade_1, grade_2, grade_3, average = 0.0

TypeError: 'float' object is not iterable