How does the @property decorator work in Python?

Question

I would like to understand how the built-in function property works. What confuses me is that property can also be used as a decorator, but it only

Answer 1

I read all the posts here and realized that we may need a real life example. Why, actually, we have @property? So, consider a Flask app where you use authentication system. You declare a model User in models.py:

class User(UserMixin, db.Model):
    __tablename__ = 'users'
    id = db.Column(db.Integer, primary_key=True)
    email = db.Column(db.String(64), unique=True, index=True)
    username = db.Column(db.String(64), unique=True, index=True)
    password_hash = db.Column(db.String(128))

    ...

    @property
    def password(self):
        raise AttributeError('password is not a readable attribute')

    @password.setter
    def password(self, password):
        self.password_hash = generate_password_hash(password)

    def verify_password(self, password):
        return check_password_hash(self.password_hash, password)

In this code we've "hidden" attribute password by using @property which triggers AttributeError assertion when you try to access it directly, while we used @property.setter to set the actual instance variable password_hash.

Now in auth/views.py we can instantiate a User with:

...
@auth.route('/register', methods=['GET', 'POST'])
def register():
    form = RegisterForm()
    if form.validate_on_submit():
        user = User(email=form.email.data,
                    username=form.username.data,
                    password=form.password.data)
        db.session.add(user)
        db.session.commit()
...

Notice attribute password that comes from a registration form when a user fills the form. Password confirmation happens on the front end with EqualTo('password', message='Passwords must match') (in case if you are wondering, but it's a different topic related Flask forms).

I hope this example will be useful