def position_to_number(func):
    """Decorator that transforms volleyball positions
    into numerical positions.

    Description
    -----------

    The usal positions on a volleyball court are:

        4    3    2
        -----------
        5    6    1

        Setter -> 1
        Outside Hitter -> 2
        Middle Blocker -> 3
        Universal -> 4
        Libero -> 6
    """
    def convert(self, position):
        if not isinstance(position, int):
            raise TypeError
        
        positions = ['Setter', 'Outside Hitter', 'Middle Blocker']

        if position == 'Universal':
            return 4
        
        if position in positions:
            return positions.index(position) + 1
                
        return None
    return convert