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Python Introducing Lists Using Lists Mutability

Joseph Michelini
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Joseph Michelini
Python Development Techdegree Graduate 18,692 Points
Posted by Joseph Michelini
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Joseph Michelini
Python Development Techdegree Graduate 18,692 Points

After reading similar questions and answers, I still don't understand why looping over the original list doesn't work.

Hey! I'm still having trouble understanding why ['sword', 'boomerang'] is left after trying to loop over the original inventory list without making a copy. I do understand how we are using the copy of the list to mutate the original list, but why can't we use inventory.remove(item) to remove all of the items in the original list?

Thanks in advance!

4 Answers

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

Modifying the same list that you are using to iterate with changes the indexes of the items. This causes the loop to skip over the next item in the sequence.

But using a copy ensures that each item will be seen by the loop.

brokemotivation
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brokemotivation
Python Development Techdegree Student 1,982 Points

Still couldn't get it. Can you please elaborate?

Tiffany Greathead
Tiffany Greathead
6,613 Points
Tiffany Greathead
Tiffany Greathead
6,613 Points

Rajat Tomar, it helped me thinking of it this way inventory = [0 "shield", 1 "sword", 2 "bow", 3 "boomerang"] (the numbers represent their index position)

When you use the "for" loop, it reassigns the index:

removes 0 (shield)

inventory = [0 "sword", 1 "bow", 2 "boomerang"]

loops again

removes 1 (bow)

inventory = [0 "sword", 1 "boomerang"]

Regardless of the change in indexes, the loop is still going to loop through each of the items in order from 0 - current len(inventory). The loop stops at one because the next index it would loop through is 2, but there is no index 2. Hope this makes sense.

Siddharth Pathak
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Siddharth Pathak
Python Development Techdegree Student 74 Points

this is the best explaination i even seen veery good tiffanygreathead

Tiffany Greathead
Tiffany Greathead
6,613 Points

Thank you Siddharth Pathak, I'm glad you found it helpful

heebers
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heebers
Python Development Techdegree Student 1,835 Points

1 year later, but thank you!!!!

cheeseslope
cheeseslope
3,303 Points
cheeseslope
cheeseslope
3,303 Points

I wrote this program to help myself visualize what’s happening here. It uses the index() method to output the removed item’s index location during the loop.

bugs = ["grasshopper", "cricket", "locust", "beetle", "spider"]

for bug in bugs:
    bug_index = bugs.index(bug)
    print(f"List is now → {bugs}")
    print("") # Blank line
    print(f"Removing '{bug}' at index {bug_index}")
    bugs.remove(bug)

print(f"List is finally → {bugs}")
print("") # Blank line
print("Bugs squashed! (Not really, though.)")

Output:

List is now → ['grasshopper', 'cricket', 'locust', 'beetle', 'spider']

Removing 'grasshopper' at index 0
List is now → ['cricket', 'locust', 'beetle', 'spider']

Removing 'locust' at index 1
List is now → ['cricket', 'beetle', 'spider']

Removing 'spider' at index 2
List is finally → ['cricket', 'beetle']

Bugs squashed! (Not really, though.)