python redis hash+set wrapper

gistfile1.py

import redis
rc = redis.Redis(port=6782)
rc.flushdb()

class HashSet(object):
    
    _rc   = None
    _rmaps = []
    
    @classmethod
    def set_redis(cls,rc):
        cls._rc = rc

    @classmethod
    def exists(cls,id):
        return cls._rc.sismember(cls._prefix+'s',id)
    
    @classmethod
    def create(cls,id):
        if cls.exists(id):
            raise KeyError(id,'already exists')
        cls._rc.sadd(cls._prefix+'s',id)
                                      
    @classmethod
    def delete(cls,id):
        if not cls.exists(id):
            raise KeyError('key not found')
        cls._rc.srem(cls._prefix+'s',id)
        # cleanup rmap first
        for field in cls._rmaps:
            rmap_index = cls._rc.hget(cls._prefix+':'+id, field)
            cls._rc.delete(field+':'+rmap_index+':'+cls._prefix)
        cls._rc.delete(cls._prefix+':'+id)
        
    @classmethod
    def instance(cls,id):
        return cls(id)
    
    @classmethod
    def rmap(cls,field,id):
        if not field in cls._fields:
            raise KeyError('invalid field')
        return cls._rc.get(field+':'+id+':'+cls._prefix)
    
    def hincrby(self, attr, count=1):
        if not attr in self.__class__._fields:
            raise KeyError('invalid field')
        if not self.__class__._fields[attr] is int:
            raise TypeError('can only increment ints')
        return self.__class__._rc.hincrby(self.__class__._prefix+':'+self._id,attr,count)

    def __init__(self,id):
        if not self.__class__.exists(id):
            raise KeyError(id,'has not been created yet')
        self.__dict__['_id'] = id

    def __getattr__(self, attr):
        if not attr in self._fields:
            raise KeyError('invalid field')
        # get value then type cast
        return self.__class__._fields[attr](self.__class__._rc.hget(self.__class__._prefix+':'+self._id, attr))
    
    def __setattr__(self, attr, value):
        if not attr in self._fields:
            raise KeyError('invalid field')
        if not isinstance(value,self._fields[attr]):
            raise TypeError('expected',self.__class__._fields[attr],'got',type(value))
        if attr in self.__class__._rmaps:
            self.__class__._rc.set(attr+':'+value+':'+self.__class__._prefix,self._id)
        self.__class__._rc.hset(self.__class__._prefix+':'+self._id, attr, value)
        
class StreamHS(HashSet):
    _prefix = 'stream'
    _fields = {'frames'          : int,
               'status'          : str,
               'error_count'     : int,
               'system_hash'     : str,
               'integrator_hash' : str,
               'download_token'  : str,
               'cc_id'           : str,
               'steps_per_frame' : int
               }
    _rmaps  = ['download_token']

# Usage example
StreamHS.set_redis(rc)

stream_id = '5lk2j345'
StreamHS.create(stream_id)
s = StreamHS.instance(stream_id)
s.frames = 5
s.status = 'OK'
s.download_token = 'bnzl2l3n1'
StreamHS.rmap('download_token','bnzl2l3n1') # returns 5lk2j345
StreamHS.delete(stream_id)