SQLAlchemy lost connection to Database when I am accessing model's property

Question

python 2.7.10 SQLAlchemy 1.2.15

I have a model like this

class SomeClass(Base):
    __tablename__ = 'some_table'
    id = Column(Integer, primary_key=True)
    name =  Column(String(50))

if there will be a lot of time between getting an object and accessing its property I can get an exception

obj = SomeClass.objects.get(11)
sleep(10000) # Some time passed
print(obj.name)

Here I can get an Exception

psycopg2.InterfaceError: connection already closed

Is there any way to avoid this Exception?

UPD: I don't understand why it needs a connection to DB when it tries to get property from obj even if obj was received before???