2018-01-02T17:20:33Z

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.

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
'pbkdf2:sha256:50000$vT9fkZM8$04dfa35c6476acf7e788a1b5b3c35e217c78dc04539d295f011f01f18cd2'

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')
True
>>> check_password_hash(hash, 'barfoo')
False

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')
False
>>> u.check_password('mypassword')
True

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
# ...

@login.user_loader
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 an 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

# ...

@app.route('/logout')
def logout():
    logout_user()
    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

    <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>

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

@app.route('/')
@app.route('/index')
@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

@app.route('/')
@app.route('/index')
@login_required
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 go to the application's / or /index URLs, 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.

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. I have added two of those methods to this class for the username and email fields. 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 an exception of type 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 %}
    <h1>Register</h1>
    <form action="" method="post">
        {{ form.hidden_tag() }}
        <p>
            {{ form.username.label }}<br>
            {{ form.username(size=32) }}<br>
            {% for error in form.username.errors %}
            <span style="color: red;">[{{ error }}]</span>
            {% endfor %}
        </p>
        <p>
            {{ form.email.label }}<br>
            {{ form.email(size=64) }}<br>
            {% for error in form.email.errors %}
            <span style="color: red;">[{{ error }}]</span>
            {% endfor %}
        </p>
        <p>
            {{ form.password.label }}<br>
            {{ form.password(size=32) }}<br>
            {% for error in form.password.errors %}
            <span style="color: red;">[{{ error }}]</span>
            {% endfor %}
        </p>
        <p>
            {{ form.password2.label }}<br>
            {{ form.password2(size=32) }}<br>
            {% for error in form.password2.errors %}
            <span style="color: red;">[{{ error }}]</span>
            {% endfor %}
        </p>
        <p>{{ form.submit() }}</p>
    </form>
{% 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)
        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)

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.

578 comments

  • #326 Miguel Grinberg said 2019-10-17T09:08:26Z

    @Fernando: posts=posts continues to work because the fake dictionary with posts is still there. It will be removed and replaced with the real thing soon.

  • #327 Jay said 2019-10-27T21:16:59Z

    For anyone following along in late 2019, I had to import model.load_user in routes.py in order to get around an error. "No user_loader has been installed for this LoginManager."

  • #328 Miguel Grinberg said 2019-10-27T23:45:14Z

    @Jay: that shouldn't be necessary, since models.py is imported at the bottom of app/init.py, My guess is that you missed that.

  • #329 Truman said 2019-10-31T00:56:10Z

    "The session is unavailable because no secret " RuntimeError: The session is unavailable because no secret key was set. Set the secret_key on the application to something unique and secret.

    Help is appreciated.

  • #330 Miguel Grinberg said 2019-10-31T09:04:34Z

    @Truman: you haven't set the SECRET_KEY variable in the configuration. Go back to chapter 3 of this tutorial to learn about this.

  • #331 Legorooj said 2019-11-01T05:09:54Z

    Ran into a problem, that you should point out to users in this tutorial;

    On startup, I was setting 'SECRET_KEY to a random value every time, but that means that the remeber_me token doesn't work after any server change.

  • #332 Miguel Grinberg said 2019-11-01T07:51:23Z

    @Legorooj: It's okay to use a random value, the problem is changing it every time you run the application. You should generate your random key and then keep using it. The problem is much bigger than what you noticed, you will not be able to keep any data in the user session if you continuously change the secret key, as this invalidates the signatures that protect the contents of the session against tampering.

  • #333 Allen Mark Brown said 2019-11-01T20:53:57Z

    Any help is greatly, greatly appreciated... I got to the end of Chapter 5. 1) server starts up correctly 2) login in screen at 127.0.0.1:5000 is correct. 3) Validated that my code matches your Git repo. However: if i enter any data, I get this error: sqlalchemy.exc.OperationalError sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) no such column: user.password_hash [SQL: SELECT user.id AS user_id, user.username AS user_username, user.email AS user_email, user.password_hash AS user_password_hash FROM user WHERE user.username = ? LIMIT ? OFFSET ?] [parameters: ('Mark', 1, 0)] (Background on this error at: http://sqlalche.me/e/e3q8)

  • #334 Miguel Grinberg said 2019-11-01T23:27:47Z

    @Allen: your database is not in sync with your database models. Try running "flask db upgrade".

  • #335 William said 2019-11-02T23:18:13Z

    Hi, Miguel I recently finished your tutorial and I am beginning to create the additional pages and database tables I need to get my app off the ground. One thing that I have been trying to figure out is how create an admin account. The admin account would be able to create "employee" accounts that have a higher access level than the user accounts you defined in this tutorial.

    I have read about the flask_user module roles_required where I can specify @roles_required('admin') in view functions. I defined 'admin' and 'user' in a tuple:

    ACCESS = { 'user': 0, 'admin': 1 }

    And referenced this in two definitions within the User class. For example, here is the admin def:

    def is_admin(self): return self.access == ACCESS['admin']

    I placed the @roles_required('admin') decorator under a specific @bp.route() view, but am getting the following error:

    AttributeError: 'Flask' object has no attribute 'user_manager'

    I believe the error is coming from app/init.py so I should put the 'user_manager' attribute there. I did some digging on Flask-User's page and on StackOverflow, but am not able to find a solution that doesn't further break the program. Do you have a solution in mind that would allow for multiple access level's?

    Thanks, William

  • #336 Miguel Grinberg said 2019-11-05T04:38:03Z

    @William: Unfortunately I don't use Flask-User myself, but based on the error my guess is that you are initializing the extension incorrectly. Either you are sending the Flask app instance as an argument in a place where you should be sending some other object, or else your Flask instance hasn't been correctly initialized with the extension, so it is missing attributes.

  • #337 Eduardo Gordilho said 2019-11-06T02:04:27Z

    Hello Miguel, First of all: Thank you very much for your work on this!

    Furthermore, I've found a workaround through the whole set_password function by using the hybrid_property and hybrid_method from sqlalchemy.ext.hybrid.

    As I'm also not using werkzeug.security, instead flask_bcrypt on its place. Here's how it looks:

    app/init.py: importing Bcrypt.

    ... # Importing Bcrypt from flask_bcrypt import Bcrypt ... # Wrapping my app within a Bcrypt object bcrypt = Bcrypt(app)

    app/models.py: Implementing bcrypt as my password hasher # ...

    # Importing my Bcrypt wrapped app from app import bcrypt # Importing Hybrid Attributes from sqlalchemy.ext.hybrid import hybrid_property, hybrid_method # Updating my User object password # OBS: I'm using the key 'password_hash' instead of 'password' also, bcause I'm using Python 3.6.9, generate_password will return a Binary, so why not store the password as a Binary instead of applying .enconde('utf-8'), just to store it as a db.String? class User(db.Model, UserMixin): # ... password_hash = db.Column(db.Binary(60)) # ... # Workaround so it's possible to assign a password by: user_object.password = <plaintext_password> @password.setter def password(self, plaintext_password): self.password_hash = bcrypt.generate_password_hash(plaintext_password) @hybrid_method def check_password(self, plaintext_password): return(bcrypt.check_password_hash(self.password_hash, plaintext_password))

    app/routes.py: Slightly changing the routine to set password of a user @app.route('/register', methods=['GET', 'POST']) # ...

    if form.validate_on_submit(): user = User(username=form.username.data, email=form.email.data, password=form.password.data) # ...

    microblog.py: Updating my flask shell to include the Bcrypted app # ... @app.shell_context_processor def make_shell_context(): return({ # ... 'bcrypt': bcrypt, })

    WOW, I think it's finally over. Anyways, do you find it appropriate?

    Thank you very much again for this 100% free and excellent tutorial!!!!

  • #338 Miguel Grinberg said 2019-11-07T08:28:12Z

    @Eduardo: Yes the hybrid method is a valid approach, I see no problems with it.

  • #339 Lucas said 2019-11-07T21:03:39Z

    Hello Miguel,

    I have an issue with the RegistrationForm class. I find that when i am at the index, and call the form.validate_username() method with form.username.data as argument I get the following error:

    sqlalchemy.exc.InterfaceError: (sqlite3.InterfaceError) Error binding parameter 0 - probably unsupported type. [SQL: SELECT user.id AS user_id, user.username AS user_username, user.password AS user_password FROM user WHERE user.username = ? LIMIT ? OFFSET ?] [parameters: (, 1, 0)]

    I am aware that in your Resgistration form class you have declared User.query.filter_by(username=username.data).first() so that you can query a string, in this case username.data. What i wanted to try is just to send form.username.data as parameter (as this is already a string) but i keep getting the mentioned error. I hope you can help me with this doubt.

    // class RegistrationForm(FlaskForm):

    username = StringField("username",validators=[InputRequired()]) password = PasswordField("password",validators=[InputRequired(),EqualTo("confirm")]) confirm = PasswordField("confirm password",validators=[InputRequired()]) def validate_username(self,name): new=User.query.filter_by(username=name).first() if new: return " USER ALREADY IN DATABASE" if not new: return " USER AVAILABLE!"

    @app.route("/",methods=["GET","POST"]) def index():

    form = RegistrationForm() if form.validate_on_submit(): name=form.username.data return form.validate_username(name) return render_template ("login.html",form=form)

    //

  • #340 Miguel Grinberg said 2019-11-07T21:31:16Z

    @Lucas: you are not supposed to call the validate_xxx methods yourself. Flask-WTF calls them as part of the validate_on_submit process.

  • #341 Guht said 2019-11-12T22:47:19Z

    Hello Miguel, First off thank you so much for your contribution to the Python and Flask communities. You have managed to create and continue to consistently create some of the most thorough and complete error-free examples available, which is no small feat in this crowded space. I have purchased your book for both myself and my co-worker, and continue to look back on your material for reference.

    I recently ran into a fairly common UX pattern, but after searching high and low, I can not seem to find any good explanations that utilize Flask-WTF. It is fairly common and expected by end-users to preserve form data after submitting a form that is invalid for one reason or another.

    I came back to this particular example to try and start small with just the "username" field being preserved after an unsuccessful login attempt, but I am not sure how to most efficiently and securely do this. I have searched over the Flask-WTF docs, and WTForms docs, and still can't manage to piece together a working example.

    As I think this would be extremely beneficial to the community at large, would you mind helping out here?

    Thanks, Miguel, and keep up the awesome work!

  • #342 Miguel Grinberg said 2019-11-13T11:51:20Z

    @Guht: you don't have to do anything other than let Flask-WTF (or Flask-Bootstrap) render the fields of your form, as shown in Chapter 3. If the form fields have any values associated with them, these are pre-populated when the form is rendered after a validation failure.

  • #343 René Nadorp said 2019-11-25T07:26:12Z

    Hi Miguel,

    Great tutorial!

    In section 5, I think the import of "redirect" is missing in "routes.py". After adding it, everything worked fine.

    Regards, R.

  • #344 Miguel Grinberg said 2019-11-25T22:52:00Z

    @René: redirect was used in chapter 3, so you should have had it already imported.

  • #345 ed said 2019-12-06T19:57:00Z

    Just in case anybody else gets the error "no user_loader has been installed for this loginmanager" from Flask, make sure that in init.py, after adding in the login.login_view & loginmanager lines, you still have at the very bottom the "from app import routes, models". This was tripping me up for a bit, but I realized this was causing the error.

  • #346 Fredrick said 2019-12-16T23:59:31Z

    Hi Miguel. Thanks for this wonderful flask tutorial. It's really helpful.

    I'm having issues with the redirection. From the code for the user login, when the user tries to access the /index page, it should redirect them to the login page for the credentials to be ascertained before granting them access to the index page. On my part, when i enter the url for the homepage on the browser, instead of redirecting me to the login page, i get a 500 internal server error and my url address looks like this:

    127.0.0.1/login?next=%2Findex.

    Kindly help. What am I missing/doing wrong?

  • #347 Miguel Grinberg said 2019-12-17T07:49:12Z

    @Fredrick: a 500 error means that your application has crashed. You need to look in the terminal window, where there is going to be a Python stack trace and error message. This will give you an idea of where you have made a mistake.

  • #348 Shafeek said 2019-12-30T03:35:40Z

    Hey Miguel, not sure if you're still seeing this in 2019 but I have a quick question.

    Do you use Flask-Session to store anything? Is there any part of your article which uses Flask-Session? I want to store some data there and then display it on the index.html page if it has data.

  • #349 Miguel Grinberg said 2019-12-31T08:10:17Z

    @Shafeek: Flask-Session is an optional extension. Flask supports session storage through the "session" context variable, which is used in this tutorial.

  • #350 Caden said 2020-01-05T20:45:06Z

    I know I'm accessing this kind of late, but I'm having a difficult with using the @login.user_loader decorator in my models file. Below I'll share the information for my build, as I set it up slightly differently. I'm aware the setup might not be optimal, but it made sense to me. I would appreciate any help though in getting this solved so my app will run. I checked to make sure all the imports aligned between your final project and all the code should be accurate too. Thanks for such a complete guide. It's such a good foundation for starters like me.

    My project layout: Microblog /migrations /templates app.py (routes are in here) config.py models.py forms.py

    My models.py: https://pste.me/#/models.py

    The error: https://pste.me/#/models-error

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