Why does my Python code only add up the price of the last item in my shopping cart?

So I am trying to make a python code that lets you add items to your shopping cart then once you type "done" it is supposed to add up the prices of each item you added to your shopping cart and then tell you the total price.

My issue is that once I type "done" it only adds up the price of the last item that I added to my shopping cart then prints it out.

shopping_cart = [""]
food = shopping_cart
#Stock
stock = {
"\nFruit\n\n"
"banana": 50,
"apple": 15,
"orange": 32,
"pear": 15,
"\nDrinks\n\n"
"apple juice": 10,
"orange juice": 10,
"pinapple juice": 10,
"milk": 15,
"\nSnacks\n\n"
"chips": 20,
"cookies": 20,
"chocolate": 25,
"lollies": 50
}
#Prices 
prices = {
#Fruit
"banana": 10,
"apple": 5,
"orange": 5,
"pear": 5,
#Drinks
"apple juice": 5,
"orange juice": 5,
"pinapple juice": 5,
"milk": 4.80,
#Snacks
"chips": 7,
"cookies": 4,
"chocolate": 6,
"lollies": 5.50
}
#Loop
while True:
item = input("\nEnter an item: ").lower()
shopping_cart.append(item)
if item == 'done':
break
if item == 'cart':
print(shopping_cart)
if item == 'stock':
for k, v in stock.items():
print(k + ":", v)
#Define the bill variable
def shopping_bill(food):
total = 0
for key in prices:
for key in stock:
for item in food:
if item == key in prices:
total += prices[key]
break 
print("Your bill total equals $", total)
print("Thank you for shopping at B&K Supermarket!")
shopping_bill(food)

There are two main problems here. The first is that you're defining food as shopping_cart before the user has entered their shopping list. Thus, it remains an empty list.

To fix this, move food = shopping_cart to after the while True loop, like so:

while True:
item = input("\nEnter an item: ").lower()
shopping_cart.append(item)
if item == 'done':
break
if item == 'cart':
print(shopping_cart)
if item == 'stock':
for k, v in stock.items():
print(k + ":", v)
food = shopping_cart

The second problem is that you're adding "\nFruit\n\n" and similar things to the dictionary up the top. You need to comment these out so the code doesn't think they're parts of the dictionary.

stock = {
#fruit
"banana": 50,
"apple": 15,
"orange": 32,
"pear": 15,
#drinks
"apple juice": 10,
"orange juice": 10,
"pinapple juice": 10,
"milk": 15,
"chips": 20,
"cookies": 20,
"chocolate": 25,
"lollies": 50
}

Entering this into the prompt with the code changes above

> banana
> apple
> done

produces

Your bill total equals $ 15
Thank you for shopping at B&K Supermarket!

Mostly the same as the other answer but checks that the stock doesn't go below 0.

shopping_cart = []
food = shopping_cart
#Stock
stock = {
#Fruit
"banana": 50,
"apple": 15,
"orange": 32,
"pear": 15,
#Drinks
"apple juice": 10,
"orange juice": 10,
"pinapple juice": 10,
"milk": 15,
#Snacks
"chips": 20,
"cookies": 20,
"chocolate": 25,
"lollies": 50
}
#Prices 
prices = {
#Fruit
"banana": 10,
"apple": 5,
"orange": 5,
"pear": 5,
#Drinks
"apple juice": 5,
"orange juice": 5,
"pinapple juice": 5,
"milk": 4.80,
#Snacks
"chips": 7,
"cookies": 4,
"chocolate": 6,
"lollies": 5.50
}
#Define the bill variable
def shopping_bill(food):
total = 0
for key in stock:
total += food.count(key) * prices[key]
print("Your bill total equals $", total)
print("Thank you for shopping at B&K Supermarket!")
#Loop
while True:
item = input("\nEnter an item: ").lower()
if item == 'done':
shopping_bill(shopping_cart)
break
if item == 'cart':
print(shopping_cart)
next
if item == 'stock':
for k, v in stock.items():
print(k + ":", v)
next
for key in stock:
if item == key:
if stock[item] > 0:
shopping_cart.append(item)  
stock[item] -= 1
print("added to cart")
else:
print("out of stock")