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Python Python Basics (Retired) Shopping List Shopping List Project

Herb Bresnan
Herb Bresnan
10,658 Points
Posted by Herb Bresnan
Herb Bresnan
10,658 Points

How to add commas between list items?

I am having trouble with the first challenge given by Kenneth Love. He asks to print the list with commas between each item. I am stumped. I know the same question has been asked, but the answers do not seem to work. This is the original code:

shopping_list = list()

print("What should we pick up at the sore today? ")
print("Enter 'DONE' to stop adding items. ")

while True:
    new_item = input("> ") 

    if new_item == "DONE":
        break

    shopping_list.append(new_item)
    print("Added: List has {} items.".format(len(shopping_list)))
    continue

print("Here's your list:")

for item in shopping_list:
    print(item)

by concatenating the comma: print(item + ',') adds a comma AFTER the items which means there is a comma where it should not be, after the last item. There should be a simple solution to this but I am not seeing it.

4 Answers

Dan Johnson
Dan Johnson
40,532 Points
Dan Johnson
Dan Johnson
40,532 Points

Basically you just need to determine if you're at the end of a list in some way to skip the last comma. You could ignore the last element in the loop:

report = ""

for item in shopping_list[:-1]:
    report += item + ", "
print(report + shopping_list[-1])

Or check if the you're at the last element:

report = ""
num_items = len(shopping_list)

for i in range(num_items):
    if i != num_items - 1:
        report += shopping_list[i] + ", "
    else:
        report += shopping_list[i]    

print(report)

If you're familiar with unpacking lists this functionality is built into the print function:

print(*shopping_list, sep=", ", end=" ")
Herb Bresnan
Herb Bresnan
10,658 Points
Herb Bresnan
Herb Bresnan
10,658 Points

Dan, Thank you. This is the first answer I (mostly) understand. 3 short questions to clear it up. In the first block, is [:-1] the index? If so, why [-1]? And why is it preceded by a colon?

Also, do you know of a good resource to learn beginner Py to use along with Treehouse?

Dan Johnson
Dan Johnson
40,532 Points
Dan Johnson
Dan Johnson
40,532 Points

List slicing has the following syntax:

my_list[start:end:steps]

So my_list[:-1] means, "Start from the beginning and go to the next to last element". Negative indexes start from the right, so -1 is the last element, -2 is the next to last element, and so on. You could also write it like this: my_list[0:-1].

As for resources, the official site has a page listing a lot of guides: Beginner's Guide

For a paid option Safari Books has just about any tech book you could want and it's subscription based so you don't have to buy the books outright: Safari Books

Luiz Edgard Oliveira
Luiz Edgard Oliveira
1,524 Points
Luiz Edgard Oliveira
Luiz Edgard Oliveira
1,524 Points

Hello Herb,

Although your original question has been already extensively answered by Dan above, I'd like to pitch in with an alternative that seems more in line with what has been discussed in this Treehouse course so far. Dan's solution is comprehensive, but to a complete beginner like myself, it was a bit hard to understand at first. What I did was to attempt to complete the first challenge by using only keywords presented to us by Kenneth in the videos up to this point in the course.

In the original code, instead of:

print("Here's your list:")
for item in shopping_list:
    print(item)

I used:

print("Here's your list:")
new_list = ', '.join(shopping_list)
print("{}".format(new_list))

This simpler solution seems to work nicely for the first challenge presented by Kenneth. If for some reason my solution isn't advisable, please let me know.

Russell Holz
Russell Holz
4,811 Points
Russell Holz
Russell Holz
4,811 Points

Thank this answer actually makes sense to me, and is in line with the logic and methods presented thus far.

Andrew Lopes
Andrew Lopes
3,190 Points
Andrew Lopes
Andrew Lopes
3,190 Points

Can't I use just use this?

print(', '.join(shopping_list))
Vanessa Van Gilder
Vanessa Van Gilder
7,034 Points

That's what I did, Andrew. It seems like the most concise way and in line with what Kenneth taught.

Herb Bresnan
Herb Bresnan
10,658 Points
Herb Bresnan
Herb Bresnan
10,658 Points

And the fog lifts. Thanks for your time.