Python库存管理问题：出库时扣减库存功能实现求助

我是Python新手，正在开发库存管理系统，其中Release_Materials函数的目标是实现出库操作时扣减对应商品的库存，但目前这个函数运行有问题，达不到预期效果。

以下是该函数的代码：

def Release_Materials():
    print("==================")
    print("Release Materials")
    print("==================")
    for item in inventory:
        item_index = int(input("what is the index of the item you would like to change? (type 0 to cancel): ")) - 1
        if item_index == "0":
            return
        if item_index < 0:
            return
        if check_valid_index(item_index) == False:
            print("")
            print("Error: The Index Does not exist! ")
            return
        else:
            break
        quantity = int(input("what is the quantity of item you would like to change? (type 0 to cancel): "))
        if quantity == "0":
            return
        if quantity < 0:
            return
        print(f"{inventory[item_index].get_name()} - OLD QUANTITY: {inventory[item_index].get_stock()}")
        inventory[item_index].update_stock(quantity)
        print(f"{inventory[item_index].get_name()} - NEW QUANTITY: {inventory[item_index].get_stock()}")
        print("Quantity Successfully Updated! ")
    else:
        print("No items added yet! ")

MenuDisplay()

内容的提问来源于stack exchange，提问作者Joven Rey Flores