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Python Python Basics All Together Now Cleaner Code Through Refactoring

Naga Bala Krishnan A
Naga Bala Krishnan A
905 Points

invalid literal for int() value?

TICKET_PRICE = 10
SERVICE_CHARGE = 2
tickets_remaining = 100  

# Create the calculate_price function. It takes number of tickets and returns num_tickets * TICKET_PRICE
def calculate_price(number_of_tickets):
    #creare a new constant for 2 dollar service charge
    # Add the service charge to the result
    return (number_of_tickets * TICKET_PRICE) + SERVICE_CHARGE

while tickets_remaining >= 1:    
    print("There are {} tickets remaining.".format(tickets_remaining))    
    name = input("What is your name:  ")
    num_tickets = input("Hello {}, How many tickets do you need?:  ".format(name))
    try:        
        num_tickets = int(num_tickets)
        if num_tickets > tickets_remaining:
            raise ValueError("There are only {} tickets remaining".format(tickets_remaining))
    except ValueError as err:
        print("Oh no. We ran into an issue. {}. Please try again.".format(err))
    else:    
        ticket_price = calculate_price(num_tickets)
        print("For {} tickets, the total price is {}".format(num_tickets, ticket_price))
        prompt_answer = input("Do you want to purchase the ticket(s)? Y/N    ")
        if prompt_answer.lower() == "y":
            print("Purchase of {} tickets confirmed. SOLD!".format(num_tickets))
            tickets_remaining -= num_tickets  
        else:
            print("Thanks for showing interest in us, {}".format(name))
print("Sorry the tickets are all sold out! :(")

When I give a string value to the question "How many tickets you need?" , i get "Oh no. We ran into an issue. invalid literal for int() with base 10: 'n'. Please try again."

Why am i getting invalid literal for int() error?

Mod Edit: Added code markdown to make code easier to read.

3 Answers

Because you can't turn a string into an int (unless the string is of a number). On this line:

num_tickets = int(num_tickets)

If the input was a number, you could turn it into an int and it would continue. If it was not a number, it would raise ValueError and the code under the except clause would run. That's why you got the message.

except ValueError as err:
    print("Oh no. We ran into an issue. {}. Please try again.".format(err))
Naga Bala Krishnan A
Naga Bala Krishnan A
905 Points

Thanks ursaminor. Appreciate your answer. I now understand it. Cheers!

andren
andren
28,503 Points

The int function takes a string that contains a literal number, like say "8" and converts it into an int of the same number 8, which is needed in order to perform math operations on it. It is not designed to deal with strings that contains letters or words.

So the reason why passing in "n" results in that message is that "n" is indeed not a valid input for int to process. This is also partly why the code is wrapped in a try block to begin with. To catch instances of people typing in something besides a number and giving them a chance to try again, instead of having the program instantly crash. So the behavior you are witnessing is not a bug, but the program working exactly as intended.

Naga Bala Krishnan A
Naga Bala Krishnan A
905 Points

Thanks, Andren. So in this case how will I be able to give that error message in a more user friendly way?

This being said shouldn't we have a second error telling the user what is wrong?

jeff Jackson
jeff Jackson
1,169 Points

I had a similar thought. It may not be pretty, but I handled it by using the find method and an if statement to search for the keyword remaining (thus differentiating the two types of error codes we tried to handle). see section of code below:

except ValueError as err: err_type = str(err) if (err_type.find("remaining") != -1): print("Enter a valid ticket value. {}. Please try again.".format(err)) else: print("Enter a valid ticket value. Please try again.")