Original list still modifies even though copy is made

Question

I have a list currentCell which contains info like coordinates and values and stuff. I then create a copy of that list using currentCellNew = currentCell.copy(). Im told that doing that and modifying the copy will NOT effect the original. But when i remove something from the copy, it also removes it from the original list.

Here is the code that removes from the copy:

#some variables exist outside of scope
        currentCell       = next(x for x in self.cells if x[0] == currentCellCoords)
        currentCellNew    = currentCell.copy()
        if currentCellNew[3] == 0:
            if **condition**:
                wallAxis = next(x[0] for x in list(self.Orientations.items()) if x[1] == "f")

#### Here is the code that removes the element from the copy #####
                currentCellNew[2].remove(next(x for x in currentCellNew[2] if x[0] == wallAxis))

Please post a [mre]. The current code won't run and reproduce your error if one were to run it as-is. — Pranav Hosangadi, Sep 30 '20 at 17:51
Does this answer your question? [Python copy a list of lists](https://stackoverflow.com/questions/28684154/python-copy-a-list-of-lists) — Pranav Hosangadi, Sep 30 '20 at 18:05
Yes it does, ShadowRanger pointed out the same solution. Thank you — JojoTheCodeDude, Sep 30 '20 at 18:06
The comment is automatically posted when we vote to close the question as a duplicate. ShadowRanger should have done this instead of adding an answer to the question. — Pranav Hosangadi, Sep 30 '20 at 18:09

score 1 · Accepted Answer · answered Sep 30 '20 at 17:52

list.copy() is a shallow copy; it's fine when the original list contains immutable objects, but it looks like you've got a list of lists, and those inner lists aren't being copied (the new outer list has references to the same inner lists as the first one).

You need to use the copy.deepcopy function to perform the original copy, thereby copying the contents, not just the top-level references:

import copy   # At top of file

...
currentCellNew    = copy.deepcopy(currentCell)

Ah i see, so list.copy() will not copy any nested lists. Thank you — JojoTheCodeDude, Sep 30 '20 at 17:59

Original list still modifies even though copy is made

1 Answers1