从列表中的单个元素中减去？

我是Python的新手学习它是如何工作的这是我制作的一个程序,我似乎无法减少数量

如何从列表中的单个元素中减去特定数字？

    import csv
    stockfile = open("Stock list.txt")
    csvf = csv.reader(stockfile,delimiter = ",")
    #Sets csvf vaiable to open text file
    #Delimiter is what seperates your values

    code = []
    item = []
    price = []
    quantity = []
    quantity = list(map(int, quantity))


    for row in csvf:
    code.append(row[0])
    item.append(row[1])
    price.append(row[2])
    quantity.append(row[3])

    usersearch = input("Please enter a 7-digit code: ")
    #Asks the user to enter the unique item codes
    while usersearch not in code:   
    print("This is not a valid 7-digit code")
    usersearch = input("Please enter a 7-digit code: ")
        if usersearch in code:               
        print("The code has been found")
        index = code.index(usersearch)  
        print("The item you have chosen is: %s"%item[index])   
        print("The price is: £%s"%price[index])                 
        print("There are %s left in stock"%quantity[index])
        quantity[index] -= 1

我想将数量[index]减少1

当我使用时,quantity[index] -= 1我得到一个错误:

TypeError:不支持的操作数类型 - =:'str'和'int'

这些是文件的内容:

0000001,PS4,300,50

0000011,Xbox One,300,50

0000111,极品飞车,40,20

0001111,正当原因3,45,24

0011111,Dualshock 4,48,30

没关系,我想通了,quantity=list(map(int, quantity))不得不在之前来quantity[index] -= 1

您可以使用扩充分配,-=:

a[4] -= 3

或者只是将减法的结果重新分配回索引(-=无论如何都是这样):

a[4] = a[4] - 3