def add_piece(self, pos_x, pos_y, piece):
        if pos_x + piece.shape[0] > self._length:
            print('Please move your piece more to the left')
            add_piece(pos_x, pos_y, piece)
        if pos_y + piece.shape[1] > self._length:
            print('Please move your piece more upwards')
            add_piece(pos_x, pos_y, piece)
        else:
            for i in range(piece.shape[0]):
                for j in range(piece.shape[1]):
                    if piece.item(i, j) != 0 and self._board[pos_x+i][pos_y+j] == 0:
                        self._board[i+pos_x][j+pos_y] = piece.item(i,j)
                    elif self._board[pos_x+i][pos_y+j] != 0 and piece.item(i,j) == 0:
                        print('Field is occupied. Please put somewhere else.')
                        add_piece(pos_x, pos_y, piece)
                        return