class Book(object):
    """Book class to represent a book in the library."""
    
    known_fields = ['title', 'authors', 'pages' ... ]
    
    def __init__(self, **kwargs):
        # internal dict to store book information
        self._info = {}
        
        for k, v in kwargs.iteritems():
            # check for typos
            Book.check_field_name(k)
            self._info[k] = v
        
        # either the title or ISBN needs to be provided to identify the book
        if not self.title and not self.ISBN:
            raise InputError('Title or ISBN needed to identify the book')
        
        self.owned = True if self.file_path != None else False
        self.tags = set()
            
    def __getattr__(self, name):
        return self._info.get(name, None)
    
    @staticmethod
    def check_field_name(field_name):
        if not field_name in Book.known_fields:
            warnings.warn('Unknown field: {0}'.format(field_name), UserWarning)