The chess piece must only be moved if it does not violate the rules of the game. For example, a chess piece can move to a valid location only if that location is not already occupied by a chess piece of the same color. Similarly, a piece can move only if it is the player's turn to move. Another rule states that a piece can only move if the resulting move does not result in check for the king of the same color. 

This pre_move_validation method is responsible for checking all the rules. If all validations pass, it calls the move method to update the move, as follows (see 4.06—model.py):

    def pre_move_validation(self, initial_pos, final_pos):
        initial_pos, final_pos = initial_pos.upper(), final_pos.upper()
        piece = self.get_piece_at(initial_pos)
        try:
            piece_at_destination = self.get_piece_at(final_pos)
        except:
            piece_at_destination = None
        if self.player_turn != piece.color:
            raise exceptions.NotYourTurn("Not " + piece.color + "'s turn!")
        enemy = ('white' if piece.color == 'black' else 'black')
        moves_available = piece.moves_available(initial_pos)
        if final_pos not in moves_available:
            raise exceptions.InvalidMove
        if self.get_all_available_moves(enemy):
            if self.will_move_cause_check(initial_pos, final_pos):
                raise exceptions.Check
        if not moves_available and self.is_king_under_check(piece.color):
            raise exceptions.CheckMate
        elif not moves_available:
            raise exceptions.Draw
        else:
            self.move(initial_pos, final_pos)
            self.update_game_statistics(
                piece, piece_at_destination, initial_pos, final_pos)
            self.change_player_turn(piece.color)

If the rules are not being followed, this code raises several exceptions, which are defined in the exceptions class as follows (see 4.06—exceptions.py):

 class Check(ChessError): pass
 class InvalidMove(ChessError): pass
 class CheckMate(ChessError): pass
 class Draw(ChessError): pass
 class NotYourTurn(ChessError): pass

We could have further coded the error classes, but we chose not to because we simply updated the name of the error class to the bottom label, which is sufficient for our current purpose. The error message is displayed from the shift method of the View class, as follows (see 4.06—view.py):

self.info_label["text"] = error.__class__.__name__