Changing values in nested lists with python

Question

Ok, with the same program previously I've now hit a problem I should have anticipated; the grid (of variable width and height) is constructed based on a code to alternate the symbols which creates a list, those lists are then stored in the grid as nested lists. below is a section of code for making the list (called line) the odd numbered width is necessary.

1 - + -
2 + - + 
3 - + -

   if gridwidth % 2 != 0: 
        for i in range(gridwidth):
            if i % 2 == 0:
                line.append('-')
            else:
                line.append('+')

Edit - sorry I didn't want to spam with code; the lines are put into the list grid below;

    grid = []
    for i in range(height):
        if i % 2 == 0:
            grid.append(line)
        else:
            grid.append(linerev)

line is then appended to grid by the range(height) and there is another block of code to handle the alternating lines which creates another list (linerev) - my problem is that because of how the grid is created, if I try to change a value in it say turn grid[0,0] into a + or -, it changes it along several rows as grid[1,0], grid[5,0] etc are all referring the the same list - is there any way to avoid this without using global variables, deep copy, or drastically revising how the grid is created? Any help would be much appreciated.

you aren't showing enough code. We need to see how you added line to grid. — Winston Ewert
– Winston Ewert, Commented Oct 2, 2011 at 3:46
If you want to avoid spamming with code, you should remove irrealvent pieces of code, not parts that directly impinge on the question at hand. — Winston Ewert
– Winston Ewert, Commented Oct 2, 2011 at 3:56
cool taken on board, I guess I thought that the construction of the lists within the lists was somehow more relevant. Thanks for your help. — user974986
– user974986, Commented Oct 2, 2011 at 4:02

Winston Ewert · Accepted Answer · 2011-10-02 03:52:57Z

3

The easiest way is to make copies of the lists when you add them:

grid = []
for i in range(height):
    if i % 2 == 0:
        grid.append(line[:])
    else:
        board.append(linerev[:])

answered Oct 2, 2011 at 3:52

Winston Ewert

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1 Comment

user974986 Over a year ago

Cheers, I hadn't even thought of doing that

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