I was attempting a recursive solution to the n queens problem. Please take a look at this code:

def solve(n,chess_arr,forbidden_cells=[]):

  print("1.",n,chess_arr,forbidden_cells)

  if n>(n*n)-len(forbidden_cells):
    return False

  elif n==1:
    printchessboard(chess_arr)
    return True

  else:
    for i in range(len(chess_arr)):
      for j in range(len(chess_arr[i])):

        print("2. ",n,(i,j),(i,j) not in forbidden_cells,forbidden_cells)

        if (i,j) not in forbidden_cells:
          chess_arr[i][j]=["Q"]
          forbidden_row=i
          forbidden_col=j
          partial_forbidden_cells=create_partial_forbidden(n,chess_arr,forbidden_row,forbidden_col,forbidden_cells)
          forbidden_cells.extend(partial_forbidden_cells)

          if solve(n-1,chess_arr,forbidden_cells):
            return True
          else:
            chess_arr[i][j]=[]
            for partial in partial_forbidden_cells:
              forbidden_cells.remove(partial)
    return False

I am facing a conceptual doubt here. While the first print statement always returns a non empty forbidden_cells list on several occasions but the second print statement always returns an empty list. I am at a loss why this is so? Could someone please explain.

My expectation: Since I am passing the forbidden_list as a parameter, I expect it to get updated with each iteration and at each iteration, I would like to be dealing with the most recent and updated forbidden_list.

Thanks for the help!