Tutorial: Roll Your Own User Authorisation Management With Flask-Login

I'll quickly go through how I did user authentication and authorisation within Flask in this project, because it's annoying having to synthesise multiple best practices into one. There are other tools, like flask-bouncer, that do this for you, but I found them far too difficult to configure in my experience. First, create your app i.e.

app = Flask(__name__)
login_manager = flask_login.LoginManager()
login_manager.init_app(app)

Decorate your user loader function, which should take a unique identifier, and return a class that extends flask_login.UserMixin. In this case I load a user from my regular user model class in SQLAlchemy, create a new object that extends from the flask_login class, and assign some values that I need to identify it by.

#This is my flask_login extension class; note the helper methods
class UserLogin(flask_login.UserMixin):
  def __init__(self):
    self.user_id = None
  def get_id(self):
    return self.user_id
  def get_role(self):
    return self.role
  def is_volunteer(self):
    return True if self.role=='volunteer' else False
  def is_customer(self):
    return True if self.role=='customer' else False
  def set_role(self, role):
    self.role = role
  def get_role(self):
    return self.role
#This is the user loader decorator
@login_manager.user_loader
def user_loader(id):
  user = models.User.query.filter_by(id=id).first()
  if not user:
    #Return blank and an anonymous user object will be instantiated
    return
  flask_user = models.UserLogin()
  flask_user.user_id = id
  flask_user.role = user.type
  return flask_user
#The route name that you will use to redirect users when they need to login, as defined by the route decorator
login_manager.login_view = 'login'

That's pretty much all there is to the actual configuration of flask_login. It supplies you with a current_user object, which is plugged into jinja2 so you can use it in your templates to render, for example, specific parts of a page depending on if your user is a customer or administrator. flask-bouncer integrates well with flask-login to provide privileges to users and only allow certain users to access certain routes, but it's too heavy handed, so I used another one I found on the net:

from flask.ext.login import current_user
from flask import url_for, redirect
from functools import wraps
def requires_roles(*roles):
  def wrapper(f):
    @wraps(f)
    def wrapped(*args, **kwargs):
      if current_user.get_role() not in roles:
        #Redirect the user to an unauthorized notice!
        return redirect(url_for('unauthorized'))
      return f(*args, **kwargs)
    return wrapped
  return wrapper

This one works really nicely. All you have to do now is add protection to your routes!

@app.route('/winning_bids')
@flask_login.login_required
@requires_roles('volunteer')
def winning_bids():
  #This route will only be accessible by Users with role type of 'volunteer'
  return render_template('winning_bids.html")

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