问如何替换文件中拆分行的一部分？

EN

with open("stock.txt", "r") as stock:#opens stock file
    for line in stock:
        line2 = line.split(",")#goes through the lines of the stock file and finds the product that matches the purchased product
        if line2[0] == GTIN:
            stock_level = int(line2[1])
            new_stock = stock_level - int(quantity)#Creates the new stock amount by taking away the recently purchased amount by the current stock
            with open("stock.txt", "a") as stock:
                      #replace line2[1] with the new_stock variable