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

Posted by Ednalyn De Dios
Ednalyn De Dios
20,259 Points

TypeError: unsupported operand type(s) for -: 'str' and 'int'

Got this error when I tried to add at the end of the list by pressing ENTER:

Traceback (most recent call last):
File "shopping_list_3.py", line 65, in <module>
add_to_list(new_item)
File "shopping_list_3.py", line 31, in add_to_list
shopping_list.insert(position-1, item)
TypeError: unsupported operand type(s) for -: 'str' and 'int' 

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(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()

2 Answers

Ryan S
Ryan S
27,276 Points
Ryan S
Ryan S
27,276 Points

Hi Ednalyn,

Nice work with the shopping list application. It is just one small syntax error that is causing your problem.

In your except block in add_to_list(), you are using an equality conditional expression, rather than an assignment operator.

When you hit Enter without inputting any value, you have assigned position to be an empty string. When the ValueError is thrown, it is not getting assigned to None, rather it is evaluating an equality comparison. So then it moves on to the .insert() statement (since position is not None) and tries to subtract 1 from an empty string.

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  #  Remove one equal sign
    if position is not None:
        shopping_list.insert(position-1, item)
    else:
        shopping_list.append(item)

    show_list()
Ednalyn De Dios
Ednalyn De Dios
20,259 Points
Ednalyn De Dios
Ednalyn De Dios
20,259 Points

@Ryan S AHHH! Thank you mucho!!! Pesty little bugs. LOL