Flask-SQLAlchemy Constructor

Question

in the Flask-SQLAlchemy tutorial, a constructor for the User model is defined:

from flask import Flask
from flask.ext.sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/test.db'
db = SQLAlchemy(app)


class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True)
    email = db.Column(db.String(120), unique=True)

    def __init__(self, username, email):
        self.username = username
        self.email = email

for a table with two columns, that might be acceptable, but what if I have tables with 10+ columns? do constructors have to be defined each time I define a new model?

Answer 1

In most cases not defining a constructor in your model class gives you the correct behavior.

Flask-SQLAlchemy's base model class (which is also SQLAlchemy's declarative base class) defines a constructor that just takes **kwargs and stores all the arguments given, so it isn't really necessary to define a constructor.

If you do need to define a constructor to do some model specific initialization, then do so as follows:

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True)
    email = db.Column(db.String(120), unique=True)

    def __init__(self, **kwargs):
        super(User, self).__init__(**kwargs)
        # do custom initialization here

By letting the base class handle the **kwargs you free yourself from the complexity of initializing the fields of the model.

Answer 2

I know this is a little old but regardless it should be useful for someone else with similar issue. If you encounter the "TypeError: init() takes exactly 1 argument (2 given)" - it means you need to provide the keywords when creating the objects to add to your database like so:

db.session.add(User(username='myname',email='my@email',password='mypassword')).

It is quite common to come across this small issue...but difficult to spot. I hope this helps.

Answer 3

You can write the constructor however you want, you'll just need to initialize each field before trying to save the object in the database.

class User(db.Model):
    ...

user = User()
user.username = 'foo'
user.email = 'foo@bar.com'
db.session.add(user)

You can initialize parameters in the constructor this way as well.

class User(db.Model):
    ...
    def __init__(self, username, email):
        self.username = username
        self.email = email
        self.password = generate_random_password()
        self.last_login = None