用Python的类变量实现单一模式

类变量紧接在类名后面定义，相当于java和c++的static变量

实例变量在__init__里定义，相当于java和c++的普通变量

class DaoCassandra():
__pool = None
__maps = {}
__cfs = {}

@classmethod
def getColumnFamilyMap(cls,cf):
"""
get ColumnFamilyMap according to columnFamily name
"""
if(cf in cls.__maps):
return cls.__maps[cf]
if(cls.__pool is None):
cls.__pool = ConnectionPool(keyspace=cls.keyspace, server_list=cls.server_list,timeout=cls.timeout)
col_fam = ColumnFamily(cls.__pool, cf)
map = ColumnFamilyMap(Msg, col_fam)
cls.__maps[cf] = map
return map