python - Decreasing value in dictionary -

i learning python , have simple problem program. have 2 dictionaries keys string , values connected them. lets it's shop fruits , prices.

shopping_list = ["banana", "orange", "apple"]  stock = {     "banana": 6,     "apple": 0,     "orange": 32,     "pear": 15 }  prices = {     "banana": 4,     "apple": 2,     "orange": 1.5,     "pear": 3 }  # function calculate bill def compute_bill(food):     total = 0      number in food:         if (stock[number]>0):             total += prices[number]             stock[number] -= 1     return total  print compute_bill(shopping_list) 

if fruit in stock add price bill , reduce amount in stock. if not, don't anything.

error message:

calling compute_bill list containing 1 apple, 1 pear , 1 banana resulted in 0 instead of correct 7

i don't know why code not working properly.

there 2 things wrong here:

• you return immediately after testing first item, ignoring rest of items. return exits function, moment python executes it.
• you decrement stock in wrong place, outside of loop.

both need correcting:

def compute_bill(food):     total = 0      item in food:         if stock[item] > 0:             # item in stock. add price total,             # , reduce stock one.             total += prices[item]             stock[item] -= 1     # loop done, return total sum of items     # in stock.     return total 

now, instead of returning total first item test, test items. , whenever have found item in stock, add price 1 item total, , reduce stock item 1.

one final detail, code academy tester calls compute_bill() function for you. don't call in code, otherwise test fails have changed stock begin with. remove print compute_fill(shopping_list) line.