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Python Python Basics (2015) Shopping List App Second Shopping List App

Freddy Venegas
Freddy Venegas
1,226 Points
Posted by Freddy Venegas
Freddy Venegas
1,226 Points

Argument in def add_to_list(): is it necessary?

When the def add_to_list(new_item): function was created I noticed that it works with or without the argument 'new_item' why is that? So, if I run my script like this,

def add_to_list(new_item):
    # add new items to list
    shopping_list.append(new_item)
    print("Added {}. List now has {} of items.".format(new_item, len(shopping_list)))

# while loop would be here
...
add_to_list(new_item)

or like this,

def add_to_list():
    # add new items to list
    shopping_list.append(new_item)
    print("Added {}. List now has {} of items.".format(new_item, len(shopping_list)))

# while loop would be here
...
add_to_list()

both work just fine. So is passing the argument really necessary?

5 Answers

jcorum
jcorum
71,830 Points
jcorum
jcorum
71,830 Points

Freddy, when I run your second version I get this:

Traceback (most recent call last):
  File "test.py", line 5, in add_to_list
    shopping_list.append(new_item)
NameError: name 'new_item' is not defined
Freddy Venegas
Freddy Venegas
1,226 Points
Freddy Venegas
Freddy Venegas
1,226 Points

You are right, I must have been running it with the argument and didn't realize. My mistake! Thank you for your looking into and response to this non-issue.

Idan Melamed
Idan Melamed
16,285 Points
Idan Melamed
Idan Melamed
16,285 Points

If I recall correctly it's a matter of scope.

You might want to try reading the next link to understand it better: http://eli.thegreenplace.net/2011/05/15/understanding-unboundlocalerror-in-python

Freddy Venegas
Freddy Venegas
1,226 Points
Freddy Venegas
Freddy Venegas
1,226 Points

Thank you Idan for this link. I shall have a read and familiarize myself as much as I can.

Simon Dein
Simon Dein
4,772 Points
Simon Dein
Simon Dein
4,772 Points

I have the same question - and my code runs fine without taking "new_item" as an argument

# making shopping list
shopping_list = []

# help function
def show_help():
    print("""
Press 'enter' to add item to shopping list
Enter 'DONE' to stop adding items and show final shopping list
Enter 'HELP' to get up this screen
Enter 'SHOW' to show what is currently on your shopping list
""")

# add items to shopping list
def add_new_item():
    shopping_list.append(new_item)
    print("{} added. {} items on list.".format(new_item, len(shopping_list)))

# show list function
def show_list():
    print("Here's your list:")
    for item in shopping_list:
        print(item)

show_help()

# ask for new additions to shopping list
while True:
    new_item = input("> ")

    # if done - loop will break
    if new_item == 'DONE':
        break

    # help for showing instructions+commands
    elif new_item == 'HELP':
        show_help()
        continue

    # show what's currently on shopping list
    elif new_item == 'SHOW':
        print(shopping_list)
        continue

    add_new_item()

# print out shopping list
show_list()
Freddy Venegas
Freddy Venegas
1,226 Points
Freddy Venegas
Freddy Venegas
1,226 Points

I believe in this case it will work without the argument because you are not calling it in def if you were to pass the argument in the function and then try to call the function without the argument you would get something like this

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: add_new_item() takes exactly 1 argument (0 given)
Simon Dein
Simon Dein
4,772 Points
Simon Dein
Simon Dein
4,772 Points

Allrighty. I would would really like a basic explaination on why he'd want to put in the argument :)

Idan Melamed
Idan Melamed
16,285 Points
Idan Melamed
Idan Melamed
16,285 Points

Hi Simon,

I think you program works because you created the variable shopping_list outside of a function at the beginning of your code, and that tells Python that it's a global variable.

Usually all your code will be written inside functions, then you will have to pass the variables you want to functions that need them.