python之从列表中的单个元素中减去

我对 Python 很陌生，了解它是如何工作的，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 quantity[index] 减 1

当我使用 quantity[index] -= 1 时出现错误:

TypeError: -=: 'str' 和 'int' 不支持的操作数类型

文件内容如下:

0000001,PS4,300,50

0000011,Xbox One,300,50

0000111,极品飞车,40,20

0001111,正当防卫 3,45,24

0011111,Dualshock 4,48,30

没关系，我想通了，quantity=list(map(int, quantity)) 必须在 quantity[index] -= 1 之前出现/p>

请您参考如下方法:

你可以使用增强赋值，-=:

a[4] -= 3 

或者只是将减法的结果重新分配回索引(无论如何 -= 都会这样做):

a[4] = a[4] - 3