在Python中与单例共享Declarative_Base（SQLAlchemy）

Question

我在一个文件里使用SQLAlchemy的时候一切都正常。现在我想把我的模型放到另一个文件里。

但是这样做不行，因为我找不到共享基础的办法。我试过用单例模式，但在model.py里它是空的，数据库里也从来没有创建出模式。

我该怎么做才能解决这个问题呢？

我的文件（简化版）:

     - /main/__init__.py
     - /main/main.py
     - /utils/__init__.py
     - /utils/utils.py
     - /model/__init__.py
     - /model/model.py

main/main.py :

from model import User
from utils.utils import readConf,createSession,getBase

class Worker(threading.Thread): 

    def __init__(self, queue, session):
        self.__queue = queue
        threading.Thread.__init__(self)
        self._stopevent = threading.Event( )

    def run(self):
        session.merge(User(queue.get()))
        session.commit()


class Collector(threading.Thread):

    def __init__(self, nom = ''):
        threading.Thread.__init__(self)
        self.nom = nom
        self._stopevent = threading.Event( )


    def run(self):
        while not self._stopevent.isSet():
            queue.put("Name")

if __name__ == '__main__':
    conf = readConf("file.")
    session = createSession(conf)
    queue = Queue.Queue(0)      
    Worker(queue, session).start()
    Collector("Start").start()

utils/utils.py

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.schema import MetaData

def createSession(conf):
    schema = conf['bdd']['type'] + '://' + conf['bdd']['user'] + ':' + conf['bdd']['password'] + '@' + conf['bdd']['host'] + '/' + conf['bdd']['db']
    engine = create_engine(schema, echo=True)

    b = getBase("Utils")
    b.set_base(declarative_base())

    Base = b.get_base()    
    Base.metadata.create_all(engine)     

    Session = sessionmaker(bind=engine)
    session = Session()
    return session

class getBase(object):
    __single = None # the one, true Singleton
    __base = None
    __text = None
    __base = None

    def __new__(cls, *args, **kwargs):
        # Check to see if a __single exists already for this class
        # Compare class types instead of just looking for None so
        # that subclasses will create their own __single objects
        if cls != type(cls.__single):
            cls.__single = object.__new__(cls, *args, **kwargs)
            __base = declarative_base()

        return cls.__single

    def __init__(self,name=None):
        self.name = name

    def get_base(self):
        return self.__base


    def set_base(self, value):
        self.__base = value

model/model.py

from utils.utils import getBase

b = getBase("model")
b.set_base(declarative_base())
Base = b.get_base()

class User(Base):
    __tablename__ = 'users'

    def __init__(self, name):
        self.screen_name = name

    name_id = Column(Integer, primary_key=True, autoincrement=True)
    screen_name = Column(BigInteger(), primary_key=True, nullable=False)

编辑 @Lafaya

我把 model/__init__.py 改成了这样 :

#!/usr/bin/python
Base = declarative_base()

然后我把 model/model.py 改成了这样 :

from model import Base

class User(Base):
    etc...

现在我不能从model.py导入Base（因为它已经被 __init.py__ 导入了）。但我需要一个对Base的引用！

Traceback (most recent call last):
  File "../main/main.py", line 12, in <module>
    from model.model import User
  File "../model/model.py", line 3, in <module>
    from model import Base
ImportError: cannot import name Base

文件结构单例模式 sqlalchemy 模型设计 python开发导入问题数据库模式共享基础

在Python中与单例共享Declarative_Base（SQLAlchemy）

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