Overwriting references

Now consider the following version of the previously shown code.

The above code outputs the following. Although we created and returned a copy of img, rgb_square was still modified. What happened?

rgb_square before flip_left_right = [[(255, 0, 0), (0, 255, 0), (0, 0, 255)],
                                     [(255, 0, 0), (0, 255, 0), (0, 0, 255)],
                                     [(255, 0, 0), (0, 255, 0), (0, 0, 255)]]

rgb_square after flip_left_right = [[(0, 0, 255), (0, 255, 0), (255, 0, 0)],
                                    [(0, 0, 255), (0, 255, 0), (255, 0, 0)],
                                    [(0, 0, 255), (0, 255, 0), (255, 0, 0)]]

Take a closer look at the following line.

rgb_square = flip_left_right(rgb_square)

Since we overwrote rgb_square with the returned value of the function flip_left_right, we overwrote the initial reference that rgb_square contained. When flip_left_right was called, both rbg_square and img contained references to the same object. When we created and returned copy_img, we returned the reference to the newly created object. We then stored this reference to rgb_square. After the above line executes, rgb_square contains the reference to the new object instead of the previous object, the original rgb_square.