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Python Python Collections (2016, retired 2019) Lists Removing items from a list

Cherie T
Cherie T
3,429 Points
Posted by Cherie T
Cherie T
3,429 Points

Can't use .remove() to remove the list from inside of messy_list. Why?

Hi there. Going through Challenge Task 2 of 2 in Python Collections on Lists.

For some reason, my code can remove the bool and str but not list... only when using messy_list.remove[1,2,3] does it remove the list. Why is that?

Thanks!

    messy_list = ["a", 2, 3, 1, False, [1, 2, 3]]

    # Your code goes below here

    messy_list.insert(0,messy_list.pop(3))


    for item in messy_list:
        if item is not int:
            messy_list.remove(item)
    messy_list.remove[1,2,3]

1 Answer

Steven Parker
Steven Parker
231,275 Points
Steven Parker
Steven Parker
231,275 Points

Removing items from an iterable while it is controlling a loop can cause other items in the loop to be skipped over. To prevent this, use a copy of the iterable to control the loop.

Also, I wouldn't expect "item is not int" to be a viable test. Did you mean to use the "type" function?

Philip Schultz
Philip Schultz
11,437 Points
Philip Schultz
Philip Schultz
11,437 Points

Hey Steven, Can you expand on this? How would you use a for loop to get rid of certain data 'types'. I can't seem to get it to work. This is what I have.

for item in copy_list:
    if item != type(int):
        messy_list.remove(item)
Steven Parker
Steven Parker
231,275 Points
Steven Parker
Steven Parker
231,275 Points

You'd want to put the "type" function on the item. And you can use the identity operator here:

    if type(item) is not int:
Philip Schultz
Philip Schultz
11,437 Points
Philip Schultz
Philip Schultz
11,437 Points

Hey Steven, Thanks for the help. When I made the adjustment to the code it deleted the bool and the string but left the list. Why is this? Also, is there a rule of thumb on when to use identity operators?

Thanks again for your help!

Steven Parker
Steven Parker
231,275 Points
Steven Parker
Steven Parker
231,275 Points

You didn't show the code where "copy_list" is created. I'm suspecting it might not be a copy but just another reference to the list and the skipping over might still be taking place.

A good example use of the identity operator would be checking my assumption (before the loop):

if copy_list is messy_list:
    print("these are both names for the SAME list")
elif copy_list == messy_list:
    print("these are copies, good")
else
    print("these are entirely different lists")
Philip Schultz
Philip Schultz
11,437 Points
Philip Schultz
Philip Schultz
11,437 Points

Here is my full code 

messy_list = ["a", 2, 3, 1, False, [1, 2, 3]]
copy_list = messy_list
print(messy_list)
for item in copy_list:
    if type(item) is not int:
        messy_list.remove(item) 
print(messy_list)

I actually haven't tried this for the treehouse challenge, I'm using my Geany editor and printing the output to my console. I'm positive that it isn't every other one though- it is printing all of the integers that are in consecutive order. The output I'm getting is [2, 3 ,1, [1, 2, 3]]

********You're right, I wasn't copying the list correctly**********************

messy_list = ["a", 2, 3, 1, False, [1, 2, 3]]
copy_list  = messy_list.copy()
print(messy_list)
for item in copy_list:
    if type(item) is not int:
        messy_list.remove(item) 
print(messy_list)
Steven Parker
Steven Parker
231,275 Points
Steven Parker
Steven Parker
231,275 Points

Yup, I guessed it, that's just a reference to the same list. You can confirm it with the little test program in the previous example. To get a copy of the list you can do this.

copy_list = messy_list.copy()  # or messy_list[:]
Philip Schultz
Philip Schultz
11,437 Points
Philip Schultz
Philip Schultz
11,437 Points

Awesome! Thanks for your help!