How to move blocks down to fill the gap after clearing a row in Tetris?

I'm working on a Tetris clone. When I clear a line, my code does this:

I want to move the squares down + 1.

My code is:

verify_val = defaultdict(list)
    for val in grid:
        x, y = val
        if (x,y) in grid:
            if grid[(x,y)] != "white":
                verify_val[y].append(x)

    # list of rows
    compare_l = [x for x in range(x_left, x_right, 1)] 
    compare_l.sort()
    lines = 0
   
    pos = []
    change = False

    pos_n = []

    for y in verify_val:
        verify_val[y].sort()
        #compare if rows are full 
        if verify_val[y] == compare_l: # it was the rows, full
            lines += 1
            for x in verify_val[y]:
                pos.append((x,y))
            change = True
        else:
            if (x,y) not in pos_n:
                for x in verify_val[y]:
                    pos_n.append((x,y))


    if change:
        for x,y in pos_n:  
            grid[(x,y)] = grid[(x,y-1)]