Are SQLAlchemy model variables class or object type?

Question

I am learning Web Development in Flask. I am using SQLAlchemy. A typical database object is structured like so:

class Role(db.Model):
    __tablename__ = 'roles'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(64), unique=True)
    default = db.Column(db.Boolean, default=False, index=True)
    permissions = db.Column(db.Integer)
    users = db.relationship('User', backref='role', lazy='dynamic')

    def __repr__(self):
        return '<Role %r>' % self.name

My question is, are these all class variables or object variables? They are outside the __init__ so it would seem they are class variables, that seems odd though. Any pointers on this would be great! Thanks!

It looks different from regular Python class code, but those are indeed instance (object) variables. Perhaps someone who knows Flask at a deeper level could give you more info about why that is. My guess it that is has to do with the object's inheritance. — coralvanda
– coralvanda, Commented Apr 17, 2017 at 7:21

Jacky Wang · Accepted Answer · 2017-05-05 06:54:07Z

8

The fields with type Column in Role are indeed class variables. But they would be replaced with InstrumentedAttribute during Role construction, which occurred in declarative.aqi.DeclarativeMeta.__init__()

This is why we define Role inherited from Base with metaclass declarative.aqi.DeclarativeMeta (the return value of declarative_base())

The InstrumentedAttribute is a data descriptor, which defined __get__ and __set__ applied to instance dictionary. As a result, we could use them to do operation through instance.

In the following example, r.name is a data descriptor, r.name = 'hello' is equivalent to Role.name.__set__(r, 'hello') -> r.__dict__['name'] = 'hello'

r = Role()
r.name = 'hello'

edited May 5, 2017 at 6:54

answered Apr 17, 2017 at 7:38

Jacky Wang

3,5403 gold badges30 silver badges50 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Are SQLAlchemy model variables class or object type?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related