Cannot use a variable in function: “Unresolved reference”

Question

I'm very new to Python (~8 hours of experience and study in a span of two days) and doing some self-created exercises to practice newly learned information. Currently focusing in classes and functions. The code is regarding selling fruits: there is a class fruit, which takes name, color, price_usd and its objects. There is also a "shopping cart" list and total_cost (initially set as 0). I have made the code work properly, however now trying to not repeat code using functions.

I created a function called "buy", which prints which fruit was chosen, adds it to the shopping cart and add its value to total_cost. However, I am getting "Unresolved reference" for total_cost (but not for shopping cart, which is also a variable outside the function?)

Error message: "UnboundLocalError: local variable 'total_cost' referenced before assignment)

(Code shown is a separate file with the problem I am trying to solve to, then, include in the main file. No new variables, though, as main file only had a lot of conditional statements and new objects.)

Tried creating total_cost before and after function. Also tried to 'manually' add fruit_choice.price_usd to total_cost: "total_cost = total_cost + fruit_choice.price_usd" It created local variable "total_cost" and gave out "Unresolved reference" for second total_cost

import fruit as fr


apple = fr.Fruits("Apple", "red", price_usd=0.88)

shopping_cart = []

# total_cost = 0


def buy(fruit_choice):

    print(f"A(n) {fruit_choice.name} has been added to your shopping cart.")

    shopping_cart.append(fruit_choice.name)
    ##############################################
    total_cost += fruit_choice.price_usd

    print(f" The total cost is: {total_cost}")
    ##############################################

total_cost = 0

print(shopping_cart, "\n", buy(apple))

(Between the ###... is where the error message is coming from.)

I expected the fruit_choice.price_usd being added to total_cost. It would be easier if that was the case, as it is the same code for every fruit.

Thank you in advance for your precious help!

Answer 1

if you would like to use a variable that was created outside of a function you need to pass it in as a parameter or use the global keyword like so:

Using global keyword:

apple = fr.Fruits("Apple", "red", price_usd=0.88)

shopping_cart = []

# total_cost = 0


def buy(fruit_choice):


    print(f"A(n) {fruit_choice.name} has been added to your shopping cart.")
    global total_cost;
    shopping_cart.append(fruit_choice.name)
    ##############################################
    total_cost += fruit_choice.price_usd

    print(f" The total cost is: {total_cost}")
    ##############################################

total_cost = 0

print(shopping_cart, "\n", buy(apple))

As parameter:

apple = fr.Fruits("Apple", "red", price_usd=0.88)

shopping_cart = []

# total_cost = 0


def buy(fruit_choice,total_cost):


    print(f"A(n) {fruit_choice.name} has been added to your shopping cart.")
    global total_cost;
    shopping_cart.append(fruit_choice.name)
    ##############################################
    total_cost += fruit_choice.price_usd
    print(f" The total cost is: {total_cost}")
    return total_cost
    ##############################################

total_cost=buy(apple)
print(shopping_cart, "\n",total_cost)

The total_cost variables needs to be created before the first call to buy() it is not important if it was created before or after definition of buy()