python - 从列表中的单个元素中减去？

Question

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>

Answer 1

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