The Flask Mega-Tutorial Part V: User Logins

This is the fifth installment of the Flask Mega-Tutorial series, in which I'm going to tell you how to create a user login subsystem.

For your reference, below is a list of the articles in this series.

Note 1: If you are looking for the legacy version of this tutorial, it's here.

Note 2: If you would like to support my work on this blog, or just don't have patience to wait for weekly articles, I am offering the complete version of this tutorial packaged as an ebook or a set of videos. For more information, visit courses.miguelgrinberg.com.

In Chapter 3 you learned how to create the user login form, and in Chapter 4 you learned how to work with a database. This chapter will teach you how to combine the topics from those two chapters to create a simple user login system.

The GitHub links for this chapter are: Browse, Zip, Diff.

Password Hashing

In Chapter 4 the user model was given a password_hash field, that so far is unused. The purpose of this field is to hold a hash of the user password, which will be used to verify the password entered by the user during the log in process. Password hashing is a complicated topic that should be left to security experts, but there are several easy to use libraries that implement all that logic in a way that is simple to be invoked from an application.

One of the packages that implement password hashing is Werkzeug, which you may have seen referenced in the output of pip when you install Flask, since it is one of its core dependencies. Since it is a dependency, Werkzeug is already installed in your virtual environment. The following Python shell session demonstrates how to hash a password:

>>> from werkzeug.security import generate_password_hash
>>> hash = generate_password_hash('foobar')
>>> hash

In this example, the password foobar is transformed into a long encoded string through a series of cryptographic operations that have no known reverse operation, which means that a person that obtains the hashed password will be unable to use it to obtain the original password. As an additional measure, if you hash the same password multiple times, you will get different results, so this makes it impossible to identify if two users have the same password by looking at their hashes.

The verification process is done with a second function from Werkzeug, as follows:

>>> from werkzeug.security import check_password_hash
>>> check_password_hash(hash, 'foobar')
>>> check_password_hash(hash, 'barfoo')

The verification function takes a password hash that was previously generated, and a password entered by the user at the time of log in. The function returns True if the password provided by the user matches the hash, or False otherwise.

The whole password hashing logic can be implemented as two new methods in the user model:

app/models.py: Password hashing and verification

from werkzeug.security import generate_password_hash, check_password_hash

# ...

class User(db.Model):
    # ...

    def set_password(self, password):
        self.password_hash = generate_password_hash(password)

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

With these two methods in place, a user object is now able to do secure password verification, without the need to ever store original passwords. Here is an example usage of these new methods:

>>> u = User(username='susan', email='susan@example.com')
>>> u.set_password('mypassword')
>>> u.check_password('anotherpassword')
>>> u.check_password('mypassword')

Introduction to Flask-Login

In this chapter I'm going to introduce you to a very popular Flask extension called Flask-Login. This extension manages the user logged-in state, so that for example users can log in to the application and then navigate to different pages while the application "remembers" that the user is logged in. It also provides the "remember me" functionality that allows users to remain logged in even after closing the browser window. To be ready for this chapter, you can start by installing Flask-Login in your virtual environment:

(venv) $ pip install flask-login

As with other extensions, Flask-Login needs to be created and initialized right after the application instance in app/__init__.py. This is how this extension is initialized:

app/__init__.py: Flask-Login initialization

# ...
from flask_login import LoginManager

app = Flask(__name__)
# ...
login = LoginManager(app)

# ...

Preparing The User Model for Flask-Login

The Flask-Login extension works with the application's user model, and expects certain properties and methods to be implemented in it. This approach is nice, because as long as these required items are added to the model, Flask-Login does not have any other requirements, so for example, it can work with user models that are based on any database system.

The four required items are listed below:

  • is_authenticated: a property that is True if the user has valid credentials or False otherwise.
  • is_active: a property that is True if the user's account is active or False otherwise.
  • is_anonymous: a property that is False for regular users, and True for a special, anonymous user.
  • get_id(): a method that returns a unique identifier for the user as a string (unicode, if using Python 2).

I can implement these four easily, but since the implementations are fairly generic, Flask-Login provides a mixin class called UserMixin that includes generic implementations that are appropriate for most user model classes. Here is how the mixin class is added to the model:

app/models.py: Flask-Login user mixin class

# ...
from flask_login import UserMixin

class User(UserMixin, db.Model):
    # ...

User Loader Function

Flask-Login keeps track of the logged in user by storing its unique identifier in Flask's user session, a storage space assigned to each user who connects to the application. Each time the logged-in user navigates to a new page, Flask-Login retrieves the ID of the user from the session, and then loads that user into memory.

Because Flask-Login knows nothing about databases, it needs the application's help in loading a user. For that reason, the extension expects that the application will configure a user loader function, that can be called to load a user given the ID. This function can be added in the app/models.py module:

app/models.py: Flask-Login user loader function

from app import login
# ...

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

The user loader is registered with Flask-Login with the @login.user_loader decorator. The id that Flask-Login passes to the function as an argument is going to be a string, so databases that use numeric IDs need to convert the string to integer as you see above.

Logging Users In

Let's revisit the login view function, which as you recall, implemented a fake login that just issued a flash() message. Now that the application has access to a user database and knows how to generate and verify password hashes, this view function can be completed.

app/routes.py: Login view function logic

# ...
from flask_login import current_user, login_user
from app.models import User

# ...

@app.route('/login', methods=['GET', 'POST'])
def login():
    if current_user.is_authenticated:
        return redirect(url_for('index'))
    form = LoginForm()
    if form.validate_on_submit():
        user = User.query.filter_by(username=form.username.data).first()
        if user is None or not user.check_password(form.password.data):
            flash('Invalid username or password')
            return redirect(url_for('login'))
        login_user(user, remember=form.remember_me.data)
        return redirect(url_for('index'))
    return render_template('login.html', title='Sign In', form=form)

The top two lines in the login() function deal with a weird situation. Imagine you have a user that is logged in, and the user navigates to the /login URL of your application. Clearly that is a mistake, so I want to not allow that. The current_user variable comes from Flask-Login and can be used at any time during the handling to obtain the user object that represents the client of the request. The value of this variable can be a user object from the database (which Flask-Login reads through the user loader callback I provided above), or a special anonymous user object if the user did not log in yet. Remember those properties that Flask-Login required in the user object? One of those was is_authenticated, which comes in handy to check if the user is logged in or not. When the user is already logged in, I just redirect to the index page.

In place of the flash() call that I used earlier, now I can log the user in for real. The first step is to load the user from the database. The username came with the form submission, so I can query the database with that to find the user. For this purpose I'm using the filter_by() method of the SQLAlchemy query object. The result of filter_by() is a query that only includes the objects that have a matching username. Since I know there is only going to be one or zero results, I complete the query by calling first(), which will return the user object if it exists, or None if it does not. In Chapter 4 you have seen that when you call the all() method in a query, the query executes and you get a list of all the results that match that query. The first() method is another commonly used way to execute a query, when you only need to have one result.

If I got a match for the username that was provided, I can next check if the password that also came with the form is valid. This is done by invoking the check_password() method I defined above. This will take the password hash stored with the user and determine if the password entered in the form matches the hash or not. So now I have two possible error conditions: the username can be invalid, or the password can be incorrect for the user. In either of those cases, I flash a message, and redirect back to the login prompt so that the user can try again.

If the username and password are both correct, then I call the login_user() function, which comes from Flask-Login. This function will register the user as logged in, so that means that any future pages the user navigates to will have the current_user variable set to that user.

To complete the login process, I just redirect the newly logged-in user to the index page.

Logging Users Out

I know I will also need to offer users the option to log out of the application. This can be done with Flask-Login's logout_user() function. Here is the logout view function:

app/routes.py: Logout view function

# ...
from flask_login import logout_user

# ...

def logout():
    return redirect(url_for('index'))

To expose this link to users, I can make the Login link in the navigation bar automatically switch to a Logout link after the user logs in. This can be done with a conditional in the base.html template:

app/templates/base.html: Conditional login and logout links

        <a href="{{ url_for('index') }}">Home</a>
        {% if current_user.is_anonymous %}
        <a href="{{ url_for('login') }}">Login</a>
        {% else %}
        <a href="{{ url_for('logout') }}">Logout</a>
        {% endif %}

The is_anonymous property is one of the attributes that Flask-Login adds to user objects through the UserMixin class. The current_user.is_anonymous expression is going to be True only when the user is not logged in.

Requiring Users To Login

Flask-Login provides a very useful feature that forces users to log in before they can view certain pages of the application. If a user who is not logged in tries to view a protected page, Flask-Login will automatically redirect the user to the login form, and only redirect back to the page the user wanted to view after the login process is complete.

For this feature to be implemented, Flask-Login needs to know what is the view function that handles logins. This can be added in app/__init__.py:

# ...
login = LoginManager(app)
login.login_view = 'login'

The 'login' value above is the function (or endpoint) name for the login view. In other words, the name you would use in a url_for() call to get the URL.

The way Flask-Login protects a view function against anonymous users is with a decorator called @login_required. When you add this decorator to a view function below the @app.route decorators from Flask, the function becomes protected and will not allow access to users that are not authenticated. Here is how the decorator can be applied to the index view function of the application:

app/routes.py: @login\_required decorator

from flask_login import login_required

def index():
    # ...

What remains is to implement the redirect back from the successful login to the page the user wanted to access. When a user that is not logged in accesses a view function protected with the @login_required decorator, the decorator is going to redirect to the login page, but it is going to include some extra information in this redirect so that the application can then return to the first page. If the user navigates to /index, for example, the @login_required decorator will intercept the request and respond with a redirect to /login, but it will add a query string argument to this URL, making the complete redirect URL /login?next=/index. The next query string argument is set to the original URL, so the application can use that to redirect back after login.

Here is a snippet of code that shows how to read and process the next query string argument:

app/routes.py: Redirect to "next" page

from flask import request
from werkzeug.urls import url_parse

@app.route('/login', methods=['GET', 'POST'])
def login():
    # ...
    if form.validate_on_submit():
        user = User.query.filter_by(username=form.username.data).first()
        if user is None or not user.check_password(form.password.data):
            flash('Invalid username or password')
            return redirect(url_for('login'))
        login_user(user, remember=form.remember_me.data)
        next_page = request.args.get('next')
        if not next_page or url_parse(next_page).netloc != '':
            next_page = url_for('index')
        return redirect(next_page)
    # ...

Right after the user is logged in by calling Flask-Login's login_user() function, the value of the next query string argument is obtained. Flask provides a request variable that contains all the information that the client sent with the request. In particular, the request.args attribute exposes the contents of the query string in a friendly dictionary format. There are actually three possible cases that need to be considered to determine where to redirect after a successful login:

  • If the login URL does not have a next argument, then the user is redirected to the index page.
  • If the login URL includes a next argument that is set to a relative path (or in other words, a URL without the domain portion), then the user is redirected to that URL.
  • If the login URL includes a next argument that is set to a full URL that includes a domain name, then the user is redirected to the index page.

The first and second cases are self-explanatory. The third case is in place to make the application more secure. An attacker could insert a URL to a malicious site in the next argument, so the application only redirects when the URL is relative, which ensures that the redirect stays within the same site as the application. To determine if the URL is relative or absolute, I parse it with Werkzeug's url_parse() function and then check if the netloc component is set or not.

Showing The Logged In User in Templates

Do you recall that way back in Chapter 2 I created a fake user to help me design the home page of the application before the user subsystem was in place? Well, the application has real users now, so I can now remove the fake user and start working with real users. Instead of the fake user I can use Flask-Login's current_user in the template:

app/templates/index.html: Pass current user to template

{% extends "base.html" %}

{% block content %}
    <h1>Hi, {{ current_user.username }}!</h1>
    {% for post in posts %}
    <div><p>{{ post.author.username }} says: <b>{{ post.body }}</b></p></div>
    {% endfor %}
{% endblock %}

And I can remove the user template argument in the view function:

app/routes.py: Do not pass user to template anymore

def index():
    # ...
    return render_template("index.html", title='Home Page', posts=posts)

This is a good time to test how the login and logout functionality works. Since there is still no user registration, the only way to add a user to the database is to do it via the Python shell, so run flask shell and enter the following commands to register a user:

>>> u = User(username='susan', email='susan@example.com')
>>> u.set_password('cat')
>>> db.session.add(u)
>>> db.session.commit()

If you start the application and try to access http://localhost:5000/ or http://localhost:5000/index, you will be immediately redirected to the login page, and after you log in using the credentials of the user that you added to your database, you will be returned to the original page, in which you will see a personalized greeting.

User Registration

The last piece of functionality that I'm going to build in this chapter is a registration form, so that users can register themselves through a web form. Let's begin by creating the web form class in app/forms.py:

app/forms.py: User registration form

from flask_wtf import FlaskForm
from wtforms import StringField, PasswordField, BooleanField, SubmitField
from wtforms.validators import ValidationError, DataRequired, Email, EqualTo
from app.models import User

# ...

class RegistrationForm(FlaskForm):
    username = StringField('Username', validators=[DataRequired()])
    email = StringField('Email', validators=[DataRequired(), Email()])
    password = PasswordField('Password', validators=[DataRequired()])
    password2 = PasswordField(
        'Repeat Password', validators=[DataRequired(), EqualTo('password')])
    submit = SubmitField('Register')

    def validate_username(self, username):
        user = User.query.filter_by(username=username.data).first()
        if user is not None:
            raise ValidationError('Please use a different username.')

    def validate_email(self, email):
        user = User.query.filter_by(email=email.data).first()
        if user is not None:
            raise ValidationError('Please use a different email address.')

There are a couple of interesting things in this new form related to validation. First, for the email field I've added a second validator after DataRequired, called Email. This is another stock validator that comes with WTForms that will ensure that what the user types in this field matches the structure of an email address.

The Email() validator from WTForms requires an external dependency to be installed.

(venv) $ pip install email-validator

Since this is a registration form, it is customary to ask the user to type the password two times to reduce the risk of a typo. For that reason I have password and password2 fields. The second password field uses yet another stock validator called EqualTo, which will make sure that its value is identical to the one for the first password field.

I have also added two methods to this class called validate_username() and validate_email(). When you add any methods that match the pattern validate_<field_name>, WTForms takes those as custom validators and invokes them in addition to the stock validators. In this case I want to make sure that the username and email address entered by the user are not already in the database, so these two methods issue database queries expecting there will be no results. In the event a result exists, a validation error is triggered by raising ValidationError. The message included as the argument in the exception will be the message that will be displayed next to the field for the user to see.

To display this form on a web page, I need to have an HTML template, which I'm going to store in file app/templates/register.html. This template is constructed similarly to the one for the login form:

app/templates/register.html: Registration template

{% extends "base.html" %}

{% block content %}
    <form action="" method="post">
        {{ form.hidden_tag() }}
            {{ form.username.label }}<br>
            {{ form.username(size=32) }}<br>
            {% for error in form.username.errors %}
            <span style="color: red;">[{{ error }}]</span>
            {% endfor %}
            {{ form.email.label }}<br>
            {{ form.email(size=64) }}<br>
            {% for error in form.email.errors %}
            <span style="color: red;">[{{ error }}]</span>
            {% endfor %}
            {{ form.password.label }}<br>
            {{ form.password(size=32) }}<br>
            {% for error in form.password.errors %}
            <span style="color: red;">[{{ error }}]</span>
            {% endfor %}
            {{ form.password2.label }}<br>
            {{ form.password2(size=32) }}<br>
            {% for error in form.password2.errors %}
            <span style="color: red;">[{{ error }}]</span>
            {% endfor %}
        <p>{{ form.submit() }}</p>
{% endblock %}

The login form template needs a link that sends new users to the registration form, right below the form:

app/templates/login.html: Link to registration page

    <p>New User? <a href="{{ url_for('register') }}">Click to Register!</a></p>

And finally, I need to write the view function that is going to handle user registrations in app/routes.py:

app/routes.py: User registration view function

from app import db
from app.forms import RegistrationForm

# ...

@app.route('/register', methods=['GET', 'POST'])
def register():
    if current_user.is_authenticated:
        return redirect(url_for('index'))
    form = RegistrationForm()
    if form.validate_on_submit():
        user = User(username=form.username.data, email=form.email.data)
        flash('Congratulations, you are now a registered user!')
        return redirect(url_for('login'))
    return render_template('register.html', title='Register', form=form)

And this view function should also be mostly self-explanatory. I first make sure the user that invokes this route is not logged in. The form is handled in the same way as the one for logging in. The logic that is done inside the if validate_on_submit() conditional creates a new user with the username, email and password provided, writes it to the database, and then redirects to the login prompt so that the user can log in.

Registration Form

With these changes, users should be able to create accounts on this application, and log in and out. Make sure you try all the validation features I've added in the registration form to better understand how they work. I am going to revisit the user authentication subsystem in a future chapter to add additional functionality such as to allow the user to reset the password if forgotten. But for now, this is enough to continue building other areas of the application.


  • #51 Jean said 2018-03-16T12:12:10Z

    In the paragraph about the User Registration would the validators for username and email need the self parameter? Due to no use in these methods would they be static or does Flask need these parameters for detection of the custom validators?

  • #52 Miguel Grinberg said 2018-03-16T16:46:25Z

    @Jean: I haven't tried making the validate methods static, so I'm not sure if that works. It largely depends on how the FlaskForm base class invokes the methods. My opinion is that they should be regular methods that take the self argument, because that is what the FlaskForm class expects.

  • #53 nvo87 said 2018-03-17T16:34:14Z

    Very usefull, realy the best tutorial. Thanks! I understand everything from this chapter, and my question is: How can I use this part of code, that realize "registration and login work", as a module next time in my projects? I mean, we have many files (forms.py, routes.py, models.py, register.html, login.html and so on) and among this files there are many imports, routes, functions, related to "registration and login work". How can I include this selection of code lines in my future project very fast and easy? What if I don't want to cut every related imports and functions one by one from your project code into mine? Maybe I have to package the code from this chapter into some module? I don't know how to say correctly, sorry. Maybe you know what I should google for or have some links?

  • #54 Miguel Grinberg said 2018-03-18T06:07:17Z

    @nvo87: in chapter 15 the authentication features are packaged as a separate blueprint that you can then copy to another application as a whole.

  • #55 NITIN GEORGE CHERIAN said 2018-03-20T00:57:08Z

    Hello Miguel,

    In the login view function, when the username or password is invalid, is it required to redirect to login page? Instead can I use the render_template function to display the login page, since we are already in the login page.

    Thanks, -Nitin

  • #56 Vaibhav Chauhan said 2018-03-20T04:21:02Z

    Hi miguelgrinberg, thanks for such a good introductory tutorial, can you please help me understanding what I am doing wrong ?

    I have following form data: csrf_token=ImJkOTlkYmVlMWQ2N2MwZGVmMGQ5MTRkZmZhODBmY2E0NWFiYTY4Y2Ei.DZIZ_w.UTdQ_hkzHXoIFS8oH8D84AMWLF8&username=vector&email=vcks%40xyz.com&password=1234&password2=1234&submit=Register

    and when I hit submit i get following traceback

    File "/Users/vaibhavchauhan/microblog/app/routes.py", line 51, in register if form.validate_on_submit(): File "/Users/vaibhavchauhan/.pyenv/versions/3.6.2/envs/flasky/lib/python3.6/site-packages/flask_wtf/form.py", line 101, in validate_on_submit return self.is_submitted() and self.validate() File "/Users/vaibhavchauhan/.pyenv/versions/3.6.2/envs/flasky/lib/python3.6/site-packages/wtforms/form.py", line 310, in validate return super(Form, self).validate(extra) File "/Users/vaibhavchauhan/.pyenv/versions/3.6.2/envs/flasky/lib/python3.6/site-packages/wtforms/form.py", line 152, in validate if not field.validate(self, extra): File "/Users/vaibhavchauhan/.pyenv/versions/3.6.2/envs/flasky/lib/python3.6/site-packages/wtforms/fields/core.py", line 204, in validate stop_validation = self._run_validation_chain(form, chain) File "/Users/vaibhavchauhan/.pyenv/versions/3.6.2/envs/flasky/lib/python3.6/site-packages/wtforms/fields/core.py", line 224, in _run_validation_chain validator(form, self)

    @app.route('/register', methods=['GET','POST']) def register(): if current_user.is_authenticated: return redirect(url_for('index')) form = RegistrationForm() if form.validate_on_submit(): user = User(username=form.username.data, email=form.email.data) user.set_password(form.password.data) db.session.add(user) db.session.commit() flash('Congratulations, you are now a registered user!') return redirect(url_for('login')) return render_template('register.html', title='Register', form=form)

  • #57 Miguel Grinberg said 2018-03-20T05:32:41Z

    @Nitin: You can render the form directly, but the standard practice is to always redirect as a response to a POST request. This is covered in more detail in a later chapter, but if you want to learn about this you can google "post/redirect/get pattern".

  • #58 Miguel Grinberg said 2018-03-20T05:40:34Z

    @Vaibhav: you have not included the error message, just the stack trace, so I can't really tell you. My guess is that there is something wrong in your form class. Compare it against mine please.

  • #59 NITIN GEORGE CHERIAN said 2018-03-20T07:12:56Z

    Hello Miguel,

    In this article, you mentioned,

    "The Flask-Login extension works with the application's user model" under the heading "Preparing The User Model for Flask-Login"

    My doubt is where have we instructed "Flask-Login" extension that the model it has to work with is "User(db.Model)" ?

  • #60 NITIN GEORGE CHERIAN said 2018-03-20T07:18:39Z

    Hello Miguel,

    In this article you mentioned:

    "Each time the logged-in user navigates to a new page, Flask-Login retrieves the ID of the user from the session, and then loads that user into memory." under the heading "User Loader Functon".

    My question: Is Flask-Login using get_id() method implemented in the UserMixin class to retrieve the ID of the user?

  • #61 Avi Rastogi said 2018-03-20T14:38:44Z

    I m getting error .. " jinja2.exceptions.UndefinedError: 'current_user' is undefined " what do to ?

  • #62 Miguel Grinberg said 2018-03-20T17:52:33Z

    @Nitin: in the @login.user_loader decorator.

  • #63 Miguel Grinberg said 2018-03-20T18:02:08Z

    @Avi: my guess is that you haven't followed the instructions to configure the Flask-Login extension.

  • #64 Peter said 2018-03-20T18:10:31Z

    Hi Miguel,

    Thanks for the great tutorial series!

    Wouldn’t it make sense to have an email verification incorporated into the login process? (Spoiler) In Chapter X we implement the password reset. I was just wondering if we shouldn’t implement a verification for the email first? So we are sure, that the user has a valid email. Do you have some source or information how to implement this verification?


    Best wishes


  • #65 Miguel Grinberg said 2018-03-20T18:14:13Z

    @Peter: email verification make sense for a lot of applications, sure. For this tutorial I did not implement it, but I have done the password reset feature, which is actually very similar. If you want to see an implementation of the email verification, my Flasky app has this: https://github.com/miguelgrinberg/flasky.

  • #66 Sam said 2018-03-28T04:20:19Z

    Thanks so much for these awesome tutorials. I ran into an issue where the "if current_user.is_authenticated:" is True despite nobody being logged in, redirects, fails to be logged in for the index, then gets locked into an infinite loop. Any ideas as to what I did wrong? Thanks in advance, Sam Rosenberg

  • #67 Joel Tang said 2018-03-28T18:42:37Z

    Thanks for all the efforts on making this tutorial! I really like it and have bought the e book to show my support via Amazon.com.

    It's been great so far, but I do have a question hoping you can answer a little bit. It involves in template jinja and backend python files.

    Correct me if I am wrong. In my mind Flask can be seen as MVC framework. routes.py has played a role as controllers and those templates are views. What I didn't understand on (maybe not understand on all of the MVC framework) is that how does views/templates know variables from python files backend?

    To elaborate, let me grab some code from your tutorial:

    <div> Microblog: <a href="{{ url_for('index') }}">Home</a> {% if current_user.is_anonymous %} <a href="{{ url_for('login') }}">Login</a> {% else %} <a href="{{ url_for('logout') }}">Logout</a> {% endif %} </div>

    Out of a sudden, I found I don't know why does the template base.html know what is current_user.

    I have a guess that it is something to do with render_template function because to me it is the only relation on controller and view. But it is not so clear to me and I barely find articles talking about the relations between python files and templates relation. And if you have talked this before, please just tell where it is and I will look for it.

    Thank you again for your great tutorial and hope you can answer my question.

  • #68 Miguel Grinberg said 2018-03-29T05:06:06Z

    @Sam: Can't really know, but you should compare your code against mine on GitHub to find what's wrong. If is_authenticated returns True, my guess is that you do have a user that is logged in. You can close your browser and open a new window to reset your user session, then for sure you will not have a logged in user.

  • #69 Miguel Grinberg said 2018-03-29T05:11:56Z

    @Joel: some variables that are defined in the Python side are exported by Flask into the Jinja context so that they can be used there as well. Flask-Login's current_user variable is one example, and the url_for() function from Flask is another.

  • #70 Tim said 2018-03-31T11:46:04Z

    Great I love this guide. I followed this using python 2.7 and everything is working great up until the user login which doesn't seem to work. Not sure if I did something wrong or if it's because I'm using the older python version.

  • #71 ghouse said 2018-04-02T09:44:09Z

    Hi miguel,

    I've developed my own web application including all chapters as mentioned in ebook. Now my team suggested me to remove user authentication from web application to achieve this I've have few doubts.

    How can i launch task without an user,Currently we are using below code to launch task from user model. Code:- current_user.launch_task('task_name','task_description') How can i get task progress without user model Code:- with tasks = current_user.get_tasks_in_progress()

    Please suggest me is there any way that i can launch task and get progress without user relation.

  • #72 Miguel Grinberg said 2018-04-03T06:07:48Z

    @ghouse: You can find your tasks directly by querying the Task model.

  • #73 Miguel Grinberg said 2018-04-03T06:11:42Z

    @Tim: Hard to know. What do you mean by "doesn't work"? Any errors that you can share?

  • #74 Paul Sutton said 2018-04-03T11:31:16Z

    Hi Miguel,

    I have followed this in the ebook up until you add a user in Flask Shell and then test the login function. The login works - it will refuse an incorrect username or password. But, once I enter the correct details I get an Internal Server Error in my browser and the following error message in my command terminal:

    File "c:\users\pablo\dropbox\python\microblog\venv\lib\site-packages\werkzeug\routing.py", line 1776, in build raise BuildError(endpoint, values, method, self) werkzeug.routing.BuildError: Could not build url for endpoint 'logout'. Did you mean 'login' instead?

    I have checked routes.py for the @app.route('/logout') and base.html where the link is and have got exactly what you have written. Do you know what the error could be?


    Paul Sutton.

  • #75 Matt said 2018-04-04T00:45:27Z


    I'm posting to hopefully help others who are stuck in a similar situation.

    I originally faced an issue when using Python2.7 that the app suddenly stopped working after Chapter 5 implementation even when running your code from the GitHub. I was encountering the following error on my terminal session: TypeError: init( ) takes exactly 1 argument (2 given).

    I was able to solve the problem by changing the line in app/init.py : 'login = LoginManager(app)'

    to two separate lines: 'login = LoginManager( )' 'login.init_app(app)'

    Alternatively, it seems I was using an outdated version of flask_login (0.1.3) as when I did a pip install flask-login --upgrade (0.4.1) your code worked perfectly without revisions.

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