Changing values in nested lists with python

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开发者_JAVA技巧 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.

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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[:])