在Python中与Singleton共享Declarative_Base（SQLAlchemy）

Question

I can run normally SQLAlchemy when everything is in one file. I now want to put my model into another file.

However, this does not work because I can not find a way to share the base. I tried with a Singleton but it is Null in model.py and the schema is never created in the database.

How can I do to fix this?

My files (simplified version) :

     - /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)

EDIT @Lafaya

I changed model/__init__.py like that :

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

Then i changed model/model.py like that :

from model import Base

class User(Base):
    etc...

Now i can't import Base from model.py (because it aleady import by __init.py__ ). But i need a reference to Base !

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

Answer 1

Write the Base in the __init__.py of model package. Then you can import it and use it.

Jut put Base in model/__init__.py .