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!

Answer 1

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'