在列表中更改列表中的元素会更改整个列 - python

Question

So im trying to get a paintboard like a coordinate system with the width of 10 and height of 5 so it looks like this represented by lists within lists eg [['x','x','x','x','x','x','x','x','x','x'],['x','x','x','x','x','x','x','x','x','x'],['x','x','x','x','x','x','x','x','x','x'],['x','x','x','x','x','x','x','x','x','x'],['x','x','x','x','x','x','x','x','x','x']]

printing it out would look like this

xxxxxxxxxx
xxxxxxxxxx
xxxxxxxxxx
xxxxxxxxxx
xxxxxxxxxx

now I want to change one element in there so it looks like this

xxcxxxxxxx
xxxxxxxxxx
xxxxxxxxxx
xxxxxxxxxx
xxxxxxxxxx

my code:

class Paintboard():

    def __init__(self, width, height):

        self.width = width
        self.height = height

        extra = []
        self.board = []
        for x in range(0, self.width):
            extra.append('x')
        for y in range(0, self.height):
            self.board.append(extra)

    def str(self):

        for x in self.board:
            print(" ".join(x))

        return ""

    def paint(self):

    self.board[0][3] = "c" # here btw x and y are switched cuz of element arrangement in self.board

test = Paintboard(10, 5)
test.paint()
test.str()

Instead of just replacing element on [3,0] it actually replaces every element in the column for some reason output:

xxxcxxxxxx
xxxcxxxxxx
xxxcxxxxxx
xxxcxxxxxx
xxxcxxxxxx

Ive been trying to find the error for a long time now in my code please help me

Answer 1

self.board.append(extra)

This line appends the same object as the row to each column. So if you change the object via one row, all other rows also see the effect.

You could make a copy of the row to append instead.

self.board.append(extra.copy())

And your code should work

Answer 2

other way: you need to change for loop,

class Paintboard():

    def __init__(self, width, height):

        self.width = width
        self.height = height

        self.board = []

        for y in range(0, self.height):
            extra = []
            for x in range(0, self.width):
                extra.append('x')
            self.board.append(extra)

    def str(self):

        for x in self.board:
            print(" ".join(x))

        return ""