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Python Python Basics All Together Now Handle Exceptions

Posted by Tsvetelin Buhlev
Tsvetelin Buhlev
1,086 Points

Trouble when using "except ValueError as err:" Console displays "invalid literal for int() with base 10: 'blue'."

Near the end of the video, the lecturer has us use an "as err". I would appreciate help on where I went wrong.

TICKET_PRICE = 10

tickets_remaining = 100  

while tickets_remaining >=1:
    name = input("What is your name?   ")

    print("There are {} tickets remaining, {}".format(tickets_remaining, name))

    tickets_wanted = (input("How many tickets would you like to purchase?  "))

    try:

        tickets_wanted = int(tickets_wanted)

        if tickets_wanted > 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:

        total_price = tickets_wanted * TICKET_PRICE

        print("The total is ${}".format(total_price))

        purchase_confirmation = input("Do you want to purchase? Y/N  ")

        tickets_remaining -= tickets_wanted

        if purchase_confirmation.lower() == "y":

            print("SOLD! {} tickets left.".format(tickets_remaining))

        else:

            print("Okay, goodbye {}. ".format(name))

print("Sorry the tickets are all sold out!!")

3 Answers

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

Hey Tsvetelin Buhlev, unfortunately, that text message is created by the Python when "blue" is presented to the int() function:

$ python
Python 3.8.5 (default, Jul 28 2020, 12:59:40) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> int('blue')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'blue'

That is one of the drawbacks to using as err, you get what the system implementation gives you.

Post back if you have more questions. Good luck!!

freddie
freddie
2,762 Points

Does this mean that by implementing the as err scenario, we made the former ValueError structure obsolete?

Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

Frederic Stein, the same raised ValueError instance still exists, except now it has an additional label defined as “err” that can be used to reference it.

Aaron Elkind
Aaron Elkind
1,393 Points
Aaron Elkind
Aaron Elkind
1,393 Points

Chris Freeman Then this answer using the "as err:" breaks the error handling and messaging if a string is input for the number of tickets. So this isn't a complete solution and very confusing because it doesn't make sense to break something to fix another problem. What would be an actual working solution that accounts for both situations?

cheeseslope
cheeseslope
3,303 Points
cheeseslope
cheeseslope
3,303 Points

This was bugging me, too. I did some research and discovered that the isnumeric() method can be used here. The method returns True if a string contains only numbers. (It’ll return False if there are letters, a decimal, or any non-numbers in the string.)

I added a second if-else statement, and now, my try-except can print both errors in a friendly way.

Here’s more information → Python String isnumeric() Method

# ==================================================
# Raise ValueError if input is not an integer.
if tickets_requested.isnumeric():
    # Input is an integer, OK to convert
    tickets_requested = int(tickets_requested)

# Input is not an integer, so raise error.
else: 
    raise ValueError("Please use number keys to enter ticket quantity.")

# ==================================================
# Raise ValueError if too many tickets are requested.
if tickets_requested > tickets_remaining:
    raise ValueError("Sorry {}, we only have {} tickets left.".format(name, tickets_remaining))

# ==================================================
# Print error
except ValueError as err:
    print(err)
freddie
freddie
2,762 Points
freddie
freddie
2,762 Points

So my code:

SERVICE_CHARGE = 2
TICKET_PRICE = 10

tickets_remaining = 100  

def price_calculation(num_tickets):
    return num_tickets * TICKET_PRICE + SERVICE_CHARGE

while tickets_remaining >= 1:
    print("Hi, there are {} tickets remaining.".format(tickets_remaining))
    name = input("What's your name? ")
    num_tickets = input("Hi {}, how many tickets would you like to buy? ".format(name))
    #ValueError
    try:
        num_tickets = int(num_tickets)
        if num_tickets > tickets_remaining:
            raise ValueError("Sorry {}, we only have {} tickets left.".format(name, tickets_remaining))
    except ValueError as err:
        print("We are having a problem here. {}".format(err))
    else:
        amount_due = price_calculation(num_tickets)
        order_confirmation = input("Would you like to buy {} tickets for $ {}  (y/n)".format(num_tickets, amount_due))
        order_confirmation = order_confirmation.lower()
        #Gather credit card information
        if order_confirmation == 'y':
            print("Thank you for your purchase")
            tickets_remaining -= num_tickets
        else:
            print("Thank you {} for your visit".format(name))
else:
    print("Sorry, we're out of tickets")

provides this failure message "We are having a problem here. invalid literal for int() with base 10: 'blue' " when confronted with the invalid "number". According to my taste "We are having a problem here." would be sufficient :)