2012-06-09T06:44:24Z

The Flask Mega-Tutorial, Part V: User Logins

(Great news! There is a new version of this tutorial!)

This is the fifth article in the series in which I document my experience writing web applications in Python using the Flask microframework.

The goal of the tutorial series is to develop a decently featured microblogging application that demonstrating total lack of originality I have decided to call microblog.

NOTE: This article was revised in September 2014 to be in sync with current versions of Python and Flask.

Here is an index of all the articles in the series that have been published to date:

Recap

In the previous chapter of the series we created our database and learned how to populate it with users and posts, but we haven't hooked up any of that into our app yet. And two chapters ago we've seen how to create web forms and left with a fully implemented login form.

In this article we are going to build on what we learned about web forms and databases and write our user login system. At the end of this tutorial our little application will register new users and log them in and out.

To follow this chapter along you need to have the microblog app as we left it at the end of the previous chapter. Please make sure the app is installed and running.

An Update Regarding the State of OpenID

It's been more than three years ago that I wrote this article. Back then OpenID seemed like a nice authentication method that was gaining a lot of traction, but in 2015 there are better alternatives, and OpenID is not as widely deployed as it used to be.

I do not have plans to update this tutorial in the near future, as I have written extensively about other authentication methods elsewhere. When you follow this tutorial keep in mind that Google, which was the most prominent OpenID provider in 2012, has dropped support for this protocol completely. My recommendation is to use a Yahoo account to test OpenID in this tutorial. I have a few personal projects that still use OpenID and I use Yahoo as a provider with good results.

As far as real-world authentication, I do not think it is a good idea to use OpenID, given the lack of support. I have a few resources for you that can help you create a more modern authentication experience:

  • My Flask book covers a traditional username and password implementation, complete with user registration, password reminders and resets.
  • My OAuth Authentication with Flask blog article describes in detail how to implement OAuth authentication, which has much wider support than OpenID. With this method you can implement "Login with Facebook" type functionality. The article demonstrates how to login with Facebook and Twitter. Others, such as Google, LinkedIn, etc. can be implemented easily with the same technique.

Configuration

As in previous chapters, we start by configuring the Flask extensions that we will use. For the login system we will use two extensions, Flask-Login and Flask-OpenID. Flask-Login will handle our users logged in state, while Flask-OpenID will provide authentication. These extensions are configured as follows (file app/__init__.py):

import os
from flask_login import LoginManager
from flask_openid import OpenID
from config import basedir

lm = LoginManager()
lm.init_app(app)
oid = OpenID(app, os.path.join(basedir, 'tmp'))

The Flask-OpenID extension requires a path to a temp folder where files can be stored. For this we provide the location of our tmp folder.

Python 3 Compatiblity

Unfortunately version 1.2.1 of Flask-OpenID (the current official version) does not work well with Python 3. Check what version you have by running the following command:

$ flask/bin/pip freeze

If you have a version newer than 1.2.1 then the problem is likely resolved, but if you have 1.2.1 and are following this tutorial on Python 3 then you have to install the development version from GitHub:

$ flask/bin/pip uninstall flask-openid
$ flask/bin/pip install git+git://github.com/mitsuhiko/flask-openid.git

Note that you need to have git installed for this to work.

Revisiting our User model

The Flask-Login extension expects certain properties and methods to be implemented in our User class. Outside of these there are no requirements for how the class has to be implemented.

Below is our Flask-Login friendly User class (file app/models.py):

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    nickname = db.Column(db.String(64), index=True, unique=True)
    email = db.Column(db.String(120), index=True, unique=True)
    posts = db.relationship('Post', backref='author', lazy='dynamic')

    @property
    def is_authenticated(self):
        return True

    @property
    def is_active(self):
        return True

    @property
    def is_anonymous(self):
        return False

    def get_id(self):
        try:
            return unicode(self.id)  # python 2
        except NameError:
            return str(self.id)  # python 3

    def __repr__(self):
        return '<User %r>' % (self.nickname)

The is_authenticated property has a misleading name. In general this method should just return True unless the object represents a user that should not be allowed to authenticate for some reason.

The is_active property should return True for users unless they are inactive, for example because they have been banned.

The is_anonymous property should return True only for fake users that are not supposed to log in to the system.

Finally, the get_id method should return a unique identifier for the user, in unicode format. We use the unique id generated by the database layer for this. Note that due to the differences in unicode handling between Python 2 and 3 we have to provide two alternative versions of this method.

User loader callback

Now we are ready to start implementing the login system using the Flask-Login and Flask-OpenID extensions.

First, we have to write a function that loads a user from the database. This function will be used by Flask-Login (file app/views.py):

@lm.user_loader
def load_user(id):
    return User.query.get(int(id))

Note how this function is registered with Flask-Login through the lm.user_loader decorator. Also remember that user ids in Flask-Login are always unicode strings, so a conversion to an integer is necessary before we can send the id to Flask-SQLAlchemy.

The login view function

Next let's update our login view function (file app/views.py):

from flask import render_template, flash, redirect, session, url_for, request, g
from flask_login import login_user, logout_user, current_user, login_required
from app import app, db, lm, oid
from .forms import LoginForm
from .models import User

@app.route('/login', methods=['GET', 'POST'])
@oid.loginhandler
def login():
    if g.user is not None and g.user.is_authenticated:
        return redirect(url_for('index'))
    form = LoginForm()
    if form.validate_on_submit():
        session['remember_me'] = form.remember_me.data
        return oid.try_login(form.openid.data, ask_for=['nickname', 'email'])
    return render_template('login.html', 
                           title='Sign In',
                           form=form,
                           providers=app.config['OPENID_PROVIDERS'])

Notice we have imported several new modules, some of which we will use later.

The changes from our previous version are very small. We have added a new decorator to our view function. The oid.loginhandler tells Flask-OpenID that this is our login view function.

At the top of the function body we check if g.user is set to an authenticated user, and in that case we redirect to the index page. The idea here is that if there is a logged in user already we will not do a second login on top.

The g global is setup by Flask as a place to store and share data during the life of a request. As I'm sure you guessed by now, we will be storing the logged in user here.

The url_for function that we used in the redirect call is defined by Flask as a clean way to obtain the URL for a given view function. If you want to redirect to the index page you may very well use redirect('/index'), but there are very good reasons to let Flask build URLs for you.

The code that runs when we get a data back from the login form is also new. Here we do two things. First we store the value of the remember_me boolean in the flask session, not to be confused with the db.session from Flask-SQLAlchemy. We've seen that the flask.g object stores and shares data though the life of a request. The flask.session provides a much more complex service along those lines. Once data is stored in the session object it will be available during that request and any future requests made by the same client. Data remains in the session until explicitly removed. To be able to do this, Flask keeps a different session container for each client of our application.

The oid.try_login call in the following line is the call that triggers the user authentication through Flask-OpenID. The function takes two arguments, the openid given by the user in the web form and a list of data items that we want from the OpenID provider. Since we defined our User class to include nickname and email, those are the items we are going to ask for.

The OpenID authentication happens asynchronously. Flask-OpenID will call a function that is registered with the oid.after_login decorator if the authentication is successful. If the authentication fails the user will be taken back to the login page.

The Flask-OpenID login callback

Here is our implementation of the after_login function (file app/views.py):

@oid.after_login
def after_login(resp):
    if resp.email is None or resp.email == "":
        flash('Invalid login. Please try again.')
        return redirect(url_for('login'))
    user = User.query.filter_by(email=resp.email).first()
    if user is None:
        nickname = resp.nickname
        if nickname is None or nickname == "":
            nickname = resp.email.split('@')[0]
        user = User(nickname=nickname, email=resp.email)
        db.session.add(user)
        db.session.commit()
    remember_me = False
    if 'remember_me' in session:
        remember_me = session['remember_me']
        session.pop('remember_me', None)
    login_user(user, remember = remember_me)
    return redirect(request.args.get('next') or url_for('index'))

The resp argument passed to the after_login function contains information returned by the OpenID provider.

The first if statement is just for validation. We require a valid email, so if an email was not provided we cannot log the user in.

Next, we search our database for the email provided. If the email is not found we consider this a new user, so we add a new user to our database, pretty much as we have learned in the previous chapter. Note that we handle the case of a missing nickname, since some OpenID providers may not have that information.

After that we load the remember_me value from the Flask session, this is the boolean that we stored in the login view function, if it is available.

Then we call Flask-Login's login_user function, to register this is a valid login.

Finally, in the last line we redirect to the next page, or the index page if a next page was not provided in the request.

The concept of the next page is simple. Let's say you navigate to a page that requires you to be logged in, but you aren't just yet. In Flask-Login you can protect views against non logged in users by adding the login_required decorator. If the user tries to access one of the affected URLs then it will be redirected to the login page automatically. Flask-Login will store the original URL as the next page, and it is up to us to return the user to this page once the login process completed.

For this to work Flask-Login needs to know what view logs users in. We can configure this in the app's module initializer (file app/__init__.py):

lm = LoginManager()
lm.init_app(app)
lm.login_view = 'login'

The g.user global

If you were paying attention, you will remember that in the login view function we check g.user to determine if a user is already logged in. To implement this we will use the before_request event from Flask. Any functions that are decorated with before_request will run before the view function each time a request is received. So this is the right place to setup our g.user variable (file app/views.py):

@app.before_request
def before_request():
    g.user = current_user

This is all it takes. The current_user global is set by Flask-Login, so we just put a copy in the g object to have better access to it. With this, all requests will have access to the logged in user, even inside templates.

The index view

In a previous chapter we left our index view function using fake objects, because at the time we did not have users or posts in our system. Well, we have users now, so let's hook that up:

@app.route('/')
@app.route('/index')
@login_required
def index():
    user = g.user
    posts = [
        { 
            'author': {'nickname': 'John'}, 
            'body': 'Beautiful day in Portland!' 
        },
        { 
            'author': {'nickname': 'Susan'}, 
            'body': 'The Avengers movie was so cool!' 
        }
    ]
    return render_template('index.html',
                           title='Home',
                           user=user,
                           posts=posts)

There are only two changes to this function. First, we have added the login_required decorator. This will ensure that this page is only seen by logged in users.

The other change is that we pass g.user down to the template, instead of the fake object we used in the past.

This is a good time to run the application.

When you navigate to http://localhost:5000 you will instead get the login page. Keep in mind that to login with OpenID you have to use the OpenID URL from your provider. You can use one of the OpenID provider links below the URL text field to generate the correct URL for you.

As part of the login process you will be redirected to your provider's web site, where you will authenticate and authorize the sharing of some information with our application (just the email and nickname that we requested, no passwords or other personal information will be exposed).

Once the login is complete you will be taken to the index page, this time as a logged in user.

Feel free to try the remember_me checkbox. With this option enabled you can close and reopen your web browser and will continue to be logged in.

Logging out

We have implemented the log in, now it's time to add the log out.

The view function for logging out is extremely simple (file app/views.py):

@app.route('/logout')
def logout():
    logout_user()
    return redirect(url_for('index'))

But we are also missing a link to logout in the template. We are going to put this link in the top navigation bar which is in the base layout (file app/templates/base.html):

<html>
  <head>
    {% if title %}
    <title>{{ title }} - microblog</title>
    {% else %}
    <title>microblog</title>
    {% endif %}
  </head>
  <body>
    <div>Microblog:
        <a href="{{ url_for('index') }}">Home</a>
        {% if g.user.is_authenticated %}
        | <a href="{{ url_for('logout') }}">Logout</a>
        {% endif %}
    </div>
    <hr>
    {% with messages = get_flashed_messages() %}
    {% if messages %}
    <ul>
    {% for message in messages %}
        <li>{{ message }} </li>
    {% endfor %}
    </ul>
    {% endif %}
    {% endwith %}
    {% block content %}{% endblock %}
  </body>
</html>

Note how easy it is to do this. We just needed to check if we have a valid user set in g.user and if we do we just add the logout link. We have also used the opportunity to use url_for in our template.

Final words

We now have a fully functioning user login system. In the next chapter we will be creating the user profile page and will be displaying user avatars on them.

In the meantime, here is the updated application code including all the changes in this article:

Download microblog-0.5.zip.

See you next time!

Miguel

217 comments

  • #76 Prabuddh said 2013-09-28T08:10:08Z

    So far so good,

    when I write this:

    https://www.google.com/accounts/o8/id how do I add my openid

    do i type my google username.. is like this:

    https://www.google.com/accounts/o8/prabuddh or https://www.google.com/accounts/o8/id prabuddh

  • #77 Miguel Grinberg said 2013-09-28T17:01:37Z

    @Prabuddh: your Google OpenID is "https://www.google.com/accounts/o8/id". Same as mine and everybody else. The username is not part of it.

  • #78 ray said 2013-09-30T18:13:48Z

    first wonderful tutorial, i love python and web dev with it has been cool so far... OK... here's the problem, when i click on the OpenID providers and hit send all it does is load the login form again... my g.user.is_authenticated() returns false... i thought it was me, so i downloaded your code and i still had the same problem when i ran yours... i feel like a complete idiot and it seems i'm the only one with the problem... please help me... thanks

  • #79 Miguel Grinberg said 2013-10-01T05:56:37Z

    @ray: The server should be doing a redirect to the OpenID provider. Just in case it isn't clear, you need to be online to use OpenID. If you are online, then I would try different providers, Google is a good one to try, the redirect should take you to a google.com page where you have to login.

  • #80 rc said 2013-10-20T19:33:23Z

    Hi, wonderful tutorial, thanks a lot for this work! One issue, is there a workaround for OpenID on my local machine behind a NAT? Seem s no matter what I do the URL for return from Open ID login is http://localhost:5000 with the error: File "microblog/app/views.py", line 66, in after_login login_user(user, remember = remember_me) File "microblog/flask/lib/python2.7/site-packages/flask_login.py", line 576, in login_user if not force and not user.is_active(): AttributeError: 'NoneType' object has no attribute 'is_active' Any advice on getting past this blocking point? TIA

  • #81 rc said 2013-10-20T21:09:20Z

    please ignore my earlier comment I had a bug somewhere that your archive sorted out for me!! thanks again for such a great tutorial and I look forwatd to the book! ~R

  • #82 John said 2013-10-21T22:41:56Z

    Hi Miguel, Thanks for your tut. I wanted to know how this would work if I wanted to store the username and passwords on my server (local host for now)? How would this work and how would this effect the rest of the tutorial? Thanks in advance. John

  • #83 Miguel Grinberg said 2013-10-22T01:24:35Z

    @John: The rest of the application would not have to change at all, but you need to implement the username/password hashing/storage/verification yourself. I'm actually working on an implementation of this (plus optional Facebook/Twitter login) for my book.

  • #84 John said 2013-10-23T04:53:24Z

    @Miguel Great, I look forward to reading your book when it's released. Can you recommend any other great resources for learning to build web applications with flask? Thanks, John

  • #85 Miguel Grinberg said 2013-10-23T06:25:45Z

    @John: I haven't read it yet, but there is a new introductory book on Flask called "Instant Flask Web Development" from Packt.

  • #86 Ahmed said 2013-10-27T10:33:20Z

    Thank you very much Miguel for taking the time to post this tutorial, you have opened my eyes to a whole new way of writing python apps, I can't thank you enough. May god bless you

  • #87 Siyuan said 2013-11-03T09:14:20Z

    Hi Miguel, I found that redirect to Google OpenID is quite slow. It seems that the try_login() function is blocking. Yahoo and AOL can still be acceptable. Is it same as you? Any advice would be appreciated.

  • #88 Miguel Grinberg said 2013-11-03T18:18:53Z

    @Siyuan: try_login() will connect to the provider's URL and then return a redirect response that will take you to Google's login page. Either one of these could be slow from where you are, but that seems more a network connection problem than an issue with the protocol.

  • #89 mahatma_dj said 2013-11-03T20:09:50Z

    Hi Miguel, again - thanks so much for this tutorial. It must have taken a lot of work to put it all together.

    My problem is that the openid login bit doesn't seem to work for me. The javascript works (fills in the url), but when i try to sign in the redirect to the provider's site doesn't happen. However, the flashed message from the "if form.validate_on_submit..." code in the login view appears, and the LOGIN_MESSAGE from flask_login ('please log in to access this page') appears, showing I've been redirected from a @login_required decorated view. Looking at terminal the request to /index is made but then redirected to /login.

    All in, it seems that oid.try_login() isn't doing its thing for some reason, which I dont understand because, as I read it, it should be called by the validate_on_submit() method - but the flashed message from the login view function is also triggered by this method and displays as it should.

    I've checked all the relevant code over and over and cant see any error. When i run the app i dont get any errors. Can you think why the try_login method might not be working? Any pointers would be much appreciated.

  • #90 Miguel Grinberg said 2013-11-03T20:29:54Z

    @mahatma_dj: when you run in debug mode Flask-OpenID dumps information to the console, see if you find anything interesting there. Also, what OpenID provider are you using? You may want to try a different one, in case the problem is with the provider. Back when I wrote this I tested Google and Yahoo most of the time.

  • #91 mahatma_dj said 2013-11-04T07:05:34Z

    Thanks for the reply Miguel. Novice question: How do I run in debug mode? Also, I noticed a previous comment about using flask 0.9 - might that be the issue? I'm not sure what version I'm using but ill check asap...

  • #92 Miguel Grinberg said 2013-11-05T05:12:03Z

    @mahatma_dj: To enable debug mode you can start the server with "app.run(debug = True)".

  • #93 Dan said 2013-11-17T03:17:59Z

    Hi, this tutorial is great. Any ideas on how to display a full list of who is logged in?

  • #94 Miguel Grinberg said 2013-11-17T19:17:16Z

    @Dan: The easiest way is by keeping the date and time of each visit in the database. I'm showing how to do this in the next chapter. If you have that in the database then you can issue a query for all the people that visited the site in the last X minutes. If instead you need the list of users that are actively using the site then it gets more complicated because the browser does not inform the server when the user closest the page. You will need some sort of heart beat done in javascript.

  • #95 Donato said 2013-11-21T19:33:28Z

    Typo: "app__init__.py"

    should be:

    app/init.py

    Thanks for these postings!!!

  • #96 Tim said 2013-11-27T00:52:02Z

    A really great tutorial here. It's an amazing feeling to implement something that before seemed like magic. And it's quite simple, I love it.

  • #97 Sid said 2014-01-02T06:54:35Z

    Hey, great tutorial. I was curious about one thing:

    "Any functions that are decorated with before_request will run before the view function each time a request is received."

    But you didn't decorate the (/login) view, so how will it know to call before_request?

  • #98 Miguel Grinberg said 2014-01-02T07:00:46Z

    @Sid: the functon decorated with before_request is called every time a request (any request) is received. The decorator is not applied to routes, it is applied to the function that you want to run before the requests.

  • #99 Jake said 2014-01-07T23:05:54Z

    I noticed in the revisited user model that nickname and email have their index property removed. Is this a requirement?

  • #100 Miguel Grinberg said 2014-01-08T06:28:45Z

    @Jake: No, it's unintentional. The index should actually remain on the model.