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Python Python Collections (2016, retired 2019) Lists Removing Items From A List

Posted by <noob />
<noob />
17,062 Points

Adding an option to let the user that what they tried to remove is not on their list

def remove_from_list(): 
    show_list()
    what_to_remove = input("What would u like to remove?\n> ")
    try:
        shopping_list.remove(what_to_remove)
    except ValueError:
        pass
    else:
        if what_to_remove not in shopping_list:
            print("{}, is not in the list, therfore u cant remove it!".format(what_to_remove)

    show_list()

i dont understand what im doing wrong here , thanks in advance for help!

3 Answers

Alex Koumparos
seal-mask
.a{fill-rule:evenodd;}techdegree
Alex Koumparos
Python Development Techdegree Student 36,887 Points
Alex Koumparos
.a{fill-rule:evenodd;}techdegree
Alex Koumparos
Python Development Techdegree Student 36,887 Points

Hi noob developer,

It looks like you're a little confused about the components of a try/else/except(/finally) block.

You've got the try part right, this is where you write the line that you expect to be able to fail. Here you are removing an item from a list. That item might not be in the list, thus it might fail (with a ValueError).

You then use except to handle the cases where the attempt from your try actually failed. In the case of removing a non-existent item from a list, this will produce a ValueError and so you handle that here. Instead of handling it, you've just written pass.

You use else to handle code that you want to run after the code in your try was successful (so in this case, the item was successfully removed). This code will not run if the try was unsuccessful. So if the item had not existed in the list, thus leading to a ValueError, the except would execute but NOT the else.

Lastly, and just for completeness, there is a finally case that executes code regardless of whether the try piece was successful.

Hope that clears everything up for you,

Cheers

Alex

<noob />
<noob />
17,062 Points

Hi alex!, thanks for ur answer, but i still don't understand couple of things so i organized my questions: 1.instead of using the keyword 'pass' what i could do?

  1. Instead of using the else statment and in the else statment using the if statment i could excute the code like this?

    def remove_from_list(): 
show_list()
what_to_remove = input("What would u like to remove?\n> ")
try:
    shopping_list.remove(what_to_remove)
except ValueError:
      if what_to_remove not in shopping_list:
           print("{}, is not in the list, therfore u cant remove it!".format(what_to_remove)

show_list()

 if I can't do this i will thank you if u can explain to me again why i cant do this

3.i can use 'if' statment in the 'except value error'?
thanks !
Alex Koumparos
seal-mask
.a{fill-rule:evenodd;}techdegree
Alex Koumparos
Python Development Techdegree Student 36,887 Points
Alex Koumparos
.a{fill-rule:evenodd;}techdegree
Alex Koumparos
Python Development Techdegree Student 36,887 Points

Hi Noob,

You're moving in the right direction. The important thing to remember with the except branch, is that this will only execute if the thing you wanted to do in the try statement failed. In your case, you're trying to remove an item from a list. If the item isn't there, you get a ValueError.

Your except branch 'catches' that ValueError. As such, you don't need the if statement inside your except. You already know that the item wasn't in the list because you're in the except branch. Therefore, you can skip straight to the code where you respond to the absence of the item, in this case, by printing the message to the user.

Sarat Chandra is quite right to say you can solve this with a simple if/else, but since you've started down this path I think you'll learn more by seeing it through, and you're soon going to start running into try/except all the time, so the sooner you get comfortable with this construct, the happier you will be.

Cheers

Alex

<noob />
<noob />
17,062 Points

i think i figure it out!

def remove_from_list(): 
show_list()
what_to_remove = input("What would u like to remove?\n> ")
try:
    shopping_list.remove(what_to_remove)
except ValueError:
    print("The item is not available in the list")

show_list()

Thanks for the help alex, i understand it now :D

Sarat Chandra
Sarat Chandra
4,898 Points
Sarat Chandra
Sarat Chandra
4,898 Points

Hello noob developer :) Instead of complicating things with a try block, this can be achieved with simple if and else blocks. Check below. If you have any more questions, leave a comment.

def del_list():
    show_list()
    del_item=input("Enter the item you wish to to delete from your shopping list. " )
    if del_item in shopping_list:
      shopping_list.remove(del_item)
      print ("Item {} has been deleted from the list".format(del_item))
      show_list()
    else:
        print("Item {} is not present in the list.".format(del_item))
        show_help