sum函数与iterable配合使用，例如：list, set等。在您的代码中，menu_stock是float，而不是iterable。要解决您的问题，请将menu_stock声明为列表，并将append声明为列表中每个stock和price的乘积。在循环之后，调用sum函数以计算total_stock_worth。
此外，您不需要调用str()方法，format()会自动为您执行此操作。

解决方案

# Create a list "menu" with 4 items
menu = ["sandwich", "burger", "fish", "chips"]
# Create a dictionary "stock" with the stock value for each item in the menu
stock = {"sandwich" : 6, "burger" : 5, "fish" : 6, "chips" : 10}
# Create a dictionary "price" with the price for each item in the menu
price = {"sandwich" : 4.50, "burger" : 6.00, "fish" : 6.50, "chips" : 3.50}

# declare menu_stock as list
menu_stock = []

# Get the values from stock and price for each menu item and multiply by eachother
for key in price:
    menu_stock.append(price[key] * stock[key])
    # Get the sum of these values
total_stock_worth = sum(menu_stock)

# Print statement with the calculated total stock worth
print("The total stock worth is £{:.2f}".format(total_stock_worth))

menu_stock（在循环中）存储float值，而sum函数要求其参数为***可迭代***。
因此，在计算price*stock s的和之前，需要累加所有乘积。

要计算total_stock_worth，请尝试：

# Create a list "menu" with 4 items
menu = ["sandwich", "burger", "fish", "chips"]
# Create a dictionary "stock" with the stock value for each item in the menu
stock = {"sandwich": 6, "burger": 5, "fish": 6, "chips": 10}
# Create a dictionary "price" with the price for each item in the menu
price = {"sandwich": 4.50, "burger": 6.00, "fish": 6.50, "chips": 3.50}

# Get the values from stock and price for each menu item and multiply by eachother
total_stock_worth = sum(price[m] * stock[m] for m in menu)

# Print statement with the calculated total stock worth
print("The total stock worth is £" + str("{:.2f}".format(total_stock_worth)))

图纸：

The total stock worth is £131.00

您有两种选择：

• 使用for-循环，并将total_stock_worth递增+=;或
• 使用带有生成器表达式的sum( )。

在您的代码中，您尝试将两者混合，但由于以下两个原因而失败：

• 对单个值而不是一系列值调用sum;
• 每次写入total_stock_worth = ...时，都将擦除先前存储在total_stock_worth中的值。

下面是两个选项：

# Create a list "menu" with 4 items
menu = ["sandwich", "burger", "fish", "chips"]
# Create a dictionary "stock" with the stock value for each item in the menu
stock = {"sandwich" : 6, "burger" : 5, "fish" : 6, "chips" : 10}
# Create a dictionary "price" with the price for each item in the menu
price = {"sandwich" : 4.50, "burger" : 6.00, "fish" : 6.50, "chips" : 3.50}

# OPTION 1: USE A FOR-LOOP AND INCREMENT MANUALLY
total_stock_worth = 0
for key in price:
    menu_stock = price[key] * stock[key]
    total_stock_worth = total_stock_worth + menu_stock

print("The total stock worth is £" + str("{:.2f}".format(total_stock_worth)))

# OPTION 2: USE SUM() WITH A GENERATOR-EXPRESSION
total_stock_worth = sum(price[key] * stock[key] for key in price)

print("The total stock worth is £" + str("{:.2f}".format(total_stock_worth)))

两个选项的输出相同：

The total stock worth is £131.00

根据个人喜好，您可以替换：

total_stock_worth = total_stock_worth + menu_stock

与：

total_stock_worth += menu_stock

其中+=可读作"增量";这两条线其实是等价的。