How can I modify a list in a list without modifying the other lists?

Question

I have a list made out of several lists (e.g. lst = [[1, 2, 3], [1, 2, 3], [1, 2, 3]]), and I created this list appending another list to it with a for loop:

for i in range(0, 3):
    lst.append([1, 2, 3])

Then I tried to modify one of those lists (lst[0].remove(1)).

But when I do that, all of the lists get modified, and the list looks like that: [[2, 3], [2, 3], [2, 3]]

Is there any way I can modify the list so it looks like [[2, 3], [1, 2, 3], [1, 2, 3]]?

I also tried to use the del function, but I didn't seem to work.

Answer 1

If you are actually creating the list in the way you state, then the removal of an item should work as expected. What I assume you you're actually doing is creating a list by appending the same list to it multiple times, i.e.,

sublist = [1, 2, 3]
mylist = []
for _ in range(3)
    mylist.append(sublist)

In this case the outer mylist just contains pointers/references to the same object, so if you modify one you modify them all.

Instead you need to make sure you append copies of the list, e.g.,

sublist = [1, 2, 3]
mylist = []
for _ in range(3):
    mylist.append(list(sublist))  # using list creates a copy

Then you'll find that:

mylist[0].remove(1)

gives

print(mylist)
[[2, 3], [1, 2, 3], [1, 2, 3]]

Note: calling your list list is not a good idea as is overwrites the built-in Python list.

P.S. - this is a common issue encountered by people starting out in Python, see, e.g., List of lists changes reflected across sublists unexpectedly and many of the linked questions.

Answer 2

this works as expected

li = []

for i in range(0, 3):
  li.append([1, 2, 3])

li[0].remove(1)

print(li)

Output:

[[2, 3], [1, 2, 3], [1, 2, 3]]