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Python Python Collections (2016, retired 2019) Lists Shopping List Take Three

Posted by Minseok Kim
Minseok Kim
2,708 Points

ValueError not caught..?

import os

shopping_list = []


def clear_screen():
    os.system("cls" if os.name == "nt" else "clear")


def show_help():
    clear_screen()
    print("What should we pick up at the store?")
    print("""
Enter 'DONE' to stop adding items.
Enter 'HELP' for this help.
Enter 'SHOW' to see your current list.
""")


def add_to_list(item):
    show_list()
    if len(shopping_list):
        position = input("Where should i add {}?\n"
                         "press ENTER to add to the end of the list\n"
                         "> ".format(item))
    else:
        position = 0

    try:
        position = abs(int(position))
    except ValueError:
        position = None
    if position is not None:
        shopping_list.insert(position-1, item)
    else:
        shopping_list.append(new_item)

    show_list()


def show_list():
    clear_screen()

    print("Here's your list:")

    index = 1
    for item in shopping_list:
        print("{}. {}".format(index, item))
        index += 1

    print ("-"*10)


show_help()

while True:
    new_item = input("> ")

    if new_item.upper() == 'DONE' or new_item.upper() == 'QUIT':
        break
    elif new_item.upper() == 'HELP':
        show_help()
        continue
    elif new_item.upper() == 'SHOW':
        show_list()
        continue
    else:
        add_to_list(new_item)

show_list()

I added orange, apple, peach, and then when i added grape with position "a", it just added grape to the end of the list.

1 Answer

Eric M
Eric M
11,545 Points
Eric M
Eric M
11,545 Points

Hi Minseok,

A ValueError is raised, but you've written an except block to handle it. 

    try:
        position = abs(int(position))
    except ValueError:
        position = None
    if position is not None:
        shopping_list.insert(position-1, item)
    else:
        shopping_list.append(new_item)

Your try block raises a ValueError because you can't cast strings to int in this manner, then your except block says to set position to None rather than terminate the program. As position is None, the conditional branch is false and we move to the else clause's branch, which runs the append method on shopping_list.

I'd like to point out here you're passing the append method new_item which is in the global scope, it's part of your while loop. It would be more consistent to instead pass item the parameter of add_to_list that you use elsewhere in this function. If add_to_list was called by another function instead of a while loop with global scope, this reference to new_item wouldn't resolve.

If you want to see something to be sure the ValueError is executing, you could make some changes to `add_to_list() like so:

def add_to_list(item):
    show_list()
    err = ""
    if len(shopping_list):
        position = input("Where should i add {}?\n"
                         "press ENTER to add to the end of the list\n"
                         "> ".format(item))
    else:
        position = 0

    try:
        position = abs(int(position))
    except ValueError:
        position = None
        err = " - added after ValueError"
    if position is not None:
        shopping_list.insert(position-1, item)
    else:
        shopping_list.append(new_item + err)

    show_list()

In your example this would add grape to the end of the list as grape - added after ValueError. Not very good to show to a user, but potential helpful during testing and debugging!

Cheers,

Eric