def get_by_specification(items, spec):
    return [x for x in items if spec.is_satisfied_by(x)]

class SetProvider(object):

    def __init__(self, alist, *args):
        self.items = alist
        self.specs = list(args)

    def __getitem__(self, index):
        return self.get_set(index)

    def get_set(self, index):
        '''
        returns items satisfied by a given specification
        '''
        return set( get_by_specification(self.items, self.specs[index]) )

    def get_set_complement(self, index):
        '''
        returns items not satisfied by a given specification
        '''
        return set(self.items) - self[index]

    def get_union(self):
        '''
        returns items that satisfy any specification
        '''
        s = set()
        for t in self:
            s.update( t )
        return s

    def get_intersection(self):
        '''
        returns items that satisfy all specifications
        '''
        s = set( self.items )
        for t in self:
            s.intersection_update( t )
        return s
    
    def get_complement(self):
        '''
        returns items not satisfied by any specification
        '''
        return set([x for x in self.items if x not in self.get_union()])

    def get_strict_complement(self):
        '''
        returns items not satisfied by all specifications
        '''
        return set([x for x in self.items if x not in self.get_intersection()])

    def get_symmetric_difference(self):
        '''
        returns items that satisfy an odd number of specifications
        '''
        s = set()
        for t in self:
            s.symmetric_difference_update( t )
        return s