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

  • #126 TheSnoo said 2018-07-13T22:03:21Z

    In the first section of Chapter 5, when I type u = User(username='susan', email='susan@example.com') I get the error message: Traceback (most recent call last): File "", line 1, in NameError: name 'User' is not defined I have went through the github and everything seems to be correct.

  • #127 Miguel Grinberg said 2018-07-14T02:29:33Z

    @TheSnoo: is this in the Python shell? Review the last section in Chapter 4, which sets up automatic importing of your models when you run the "flask shell" command.

  • #128 Tibor said 2018-07-18T17:33:41Z

    Hi Miguel!

    Congrats for the whole tutorial! This is the best I found on the web! :) You use this 'username=username.data' pattern in some places, for example in this method:

    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.')

    Why did you use 'username=username.data'? This method gets the username params. I understand username.data is the form field. Why username.data is not enough?

    Sorry, if the question is silly, but I new in Python and I didn't see this pattern is java or Clojure.

    Regards, Tibor

  • #129 Miguel Grinberg said 2018-07-18T18:39:44Z

    @Tibor: the filter_by() method uses keyword arguments so that it knows by which field you want to do the filtering. You may want to read the Python docs regarding positional and keyword arguments, this is an area where Python differs from many other languages, keyword arguments are a very useful feature in the language.

  • #130 Tibor said 2018-07-18T19:25:22Z

    Hi Miguel!

    Sorry! That was a really silly question. I got it! So, left side of the equation is the db or model field and the right side is the parameter/variable and the method parameter is the form field, if I'm right.

    Thanks a lot!

    Regards, Tibor

  • #131 Miguel Grinberg said 2018-07-19T15:28:49Z

    @Tibor: I think you are overcomplicating this. If you want filter your database table and obtain only the entries that have the username column set to "foo", then do filter(username="foo"). Replace "foo" with any Python expression that returns the value that you want to filter on.

  • #132 Daniel Berns said 2018-07-21T03:12:57Z

    Hello Miguel. Your tutorials are wonderful. You are an excellent teacher.

    I have a question for you. I am developing a small website, with two classes of users: some few registered known users (login required for managing content) and several anonymous users (login not required for accessing content). This website must be public (We need to view some results from remote places). I dont want to use a public registering page. I think to use a small script for registering users. What do you think about this? What is the best practice?

  • #133 Miguel Grinberg said 2018-07-21T06:31:06Z

    @Daniel: you can create a custom sub-command for the flask command. You'll learn how to do that later in this tutorial.

  • #134 Shwetanshu Raj said 2018-07-24T17:59:08Z

    Hey Miguel,

    Whenever I try to run my main program it gives the following error (I have put your init.py and routes.py in a single file named main.py):

    Traceback (most recent call last): File "main.py", line 3, in from forms import LoginForm, RegistrationForm File "C:\Users\Shwetanshu\Desktop\Version1\forms.py", line 4, in from models import User File "C:\Users\Shwetanshu\Desktop\Version1\models.py", line 2, in from main import db,login File "C:\Users\Shwetanshu\Desktop\Version1\main.py", line 3, in from forms import LoginForm, RegistrationForm ImportError: cannot import name 'LoginForm'

  • #135 Miguel Grinberg said 2018-07-24T21:07:32Z

    @Shwetanshu: you have circular dependencies in your application: main imports forms, which imports models, which imports main. Solve that cycle and you should be fine.

  • #136 nick said 2018-07-26T14:54:25Z

    Hi Miguel once again thanks for this great tutorials but i run into the below error kindly help resolve

    File "/home/kino/Desktop/microblog/microblogproject/lib/python3.6/site-packages/flask/app.py", line 2309, in __call__ return self.wsgi_app(environ, start_response) File "/home/kino/Desktop/microblog/microblogproject/lib/python3.6/site-packages/flask/app.py", line 2295, in wsgi_app response = self.handle_exception(e) File "/home/kino/Desktop/microblog/microblogproject/lib/python3.6/site-packages/flask/app.py", line 1741, in handle_exception reraise(exc_type, exc_value, tb) File "/home/kino/Desktop/microblog/microblogproject/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise raise value File "/home/kino/Desktop/microblog/microblogproject/lib/python3.6/site-packages/flask/app.py", line 2292, in wsgi_app response = self.full_dispatch_request() File "/home/kino/Desktop/microblog/microblogproject/lib/python3.6/site-packages/flask/app.py", line 1815, in full_dispatch_request rv = self.handle_user_exception(e) File "/home/kino/Desktop/microblog/microblogproject/lib/python3.6/site-packages/flask/app.py", line 1718, in handle_user_exception reraise(exc_type, exc_value, tb) File "/home/kino/Desktop/microblog/microblogproject/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise raise value File "/home/kino/Desktop/microblog/microblogproject/lib/python3.6/site-packages/flask/app.py", line 1813, in full_dispatch_request rv = self.dispatch_request() File "/home/kino/Desktop/microblog/microblogproject/lib/python3.6/site-packages/flask/app.py", line 1799, in dispatch_request return self.view_functions[rule.endpoint](**req.view_args) File "/home/kino/Desktop/microblog/app/routes.py", line 57, in register if form.validate_on_submit(): File "/home/kino/Desktop/microblog/microblogproject/lib/python3.6/site-packages/flask_wtf/form.py", line 101, in validate_on_submit return self.is_submitted() and self.validate() File "/home/kino/Desktop/microblog/microblogproject/lib/python3.6/site-packages/wtforms/form.py", line 310, in validate return super(Form, self).validate(extra) File "/home/kino/Desktop/microblog/microblogproject/lib/python3.6/site-packages/wtforms/form.py", line 152, in validate if not field.validate(self, extra): File "/home/kino/Desktop/microblog/microblogproject/lib/python3.6/site-packages/wtforms/fields/core.py", line 206, in validate stop_validation = self._run_validation_chain(form, chain) File "/home/kino/Desktop/microblog/microblogproject/lib/python3.6/site-packages/wtforms/fields/core.py", line 226, in _run_validation_chain [Open an interactive python shell in this frame] validator(form, self)

    TypeError: init() takes from 1 to 2 positional arguments but 3 were given

  • #137 Miguel Grinberg said 2018-07-26T16:23:12Z

    @nick: My guess is that you have at least one validator in your registration form that was created as a class instead of an object. Note that you have to say "validators=[DataRequired()]" and not "validators=[DataRequired]".

  • #138 Carl said 2018-07-28T21:06:17Z

    Hello there, I'm kind of stuck in this chapter. When i launch my flask app, it can't open and it says there are infinite redirects. Please could some one help me out? I'm kind of stuck, and I haven't quite figured out what the issue is

  • #139 nick said 2018-07-29T05:38:09Z

    Thank you, Miguel, i found the bug was in password = PasswordField('Password', validators=[DataRequired]) as a class and not its instance, on adding parethesis everything worked very fine password = PasswordField('Password', validators=[DataRequired()])

    once again thank you.

  • #140 J C said 2018-07-30T05:50:52Z

    Hmm, I'm having an import error on LoginManager. Pycharm tells me that it cannot find reference 'LoginManager' in 'init.py'. It doesn't have an issue with from flask_login, only what comes after. This is true in every time I import something from flask_login as well, even in other files.

    When I run the flask app, I get the following traceback:

    Error: While importing "microblog", an ImportError was raised: Traceback (most recent call last): File "/home/royalcows9/PycharmProjects/FlaskTestGithub/venv/lib/python3.5/site-packages/flask/cli.py", line 235, in locate_app __import__(module_name) File "/home/royalcows9/PycharmProjects/FlaskTestGithub/microblog.py", line 1, in <module> from app import app, db File "/home/royalcows9/PycharmProjects/FlaskTestGithub/app/__init__.py", line 5, in <module> from flask_login import LoginManager ImportError: cannot import name 'LoginManager'

    I am thoroughly confused. Can someone please give me some input? Thanks!!

  • #141 Miguel Grinberg said 2018-07-30T20:49:05Z

    @Carl: impossible to help you without seeing the actual error. Compare your code against mine to see where you made the mistake.

  • #142 Miguel Grinberg said 2018-07-31T09:12:49Z

    @JC: start by recreating your virtual environment, in case your current one is corrupted. If that does not help, open a Python shell and type "from flask_login import LoginManager" to check if you can run the import directly.

  • #143 Guillaume said 2018-07-31T17:59:43Z

    Hello Miguel. I'm having fun learning flask and python with your tutorial (I'm not sure why I'm doing this, I have too much free time, probably. Learning Flask is like playing with lego to me).

    I see you are actively answering questions here, so I'm got another question for you ! How to produce custom error messages for an invalid email or for an invalid password2 ? I couldn't find an answer on my own (I'm very much a beginner) and it's bugging me.

    At this point, an invalid email will raise an error, thanks to wtforms.validators.Email(), and a wrong password2 as well, thanks to wtforms.validators.EqualTo(), but these errors don't seem to produce a message, and the app shows empty red brackets.

    Thanks for your very nice tutorial and for your help.

  • #144 Carl said 2018-07-31T20:16:37Z

    Hi Miguel. Thanks for your reply. I already compared your code against mine, and it's the same. I even ran your code, and i get the same error.

    The url above is a link to a folder on my google drive which contains the error screens.

    Thanks again.

  • #145 Miguel Grinberg said 2018-07-31T21:21:46Z

    @Guillaume: I suspect you have a problem in your template that is not rendering the error messages, but in any case, if you want to use a non-default error message for a validator, just pass the text as the first argument. For example: validators=[Email('the email is invalid')].

  • #146 Miguel Grinberg said 2018-07-31T21:49:26Z

    @Carl: the handling of the GET request to /login is incorrect in your example. Instead of a redirect, that request should display the login form. You may need to debug that to see why it does not do that. I don't see how my code could have the same behavior. Make sure the code and the template are identical to mine, pretty sure you have something in there that is different.

  • #147 Guillaume said 2018-07-31T22:20:09Z

    @Miguel: (follow-up about error messages). I already tried adding a custom message as argument of the Email() function, as I read in the wtforms doc, but it still didn't work. So yeah it was on my end.

    My problem was an extra "s" in these two lines: {% for errors in form.password2.errors %} {% for errors in form.email.errors %}

    Not sure why it didn't produce an exception, it took me a while to spot it haha.

    Thanks anyway.

  • #148 Miguel Grinberg said 2018-08-01T22:35:26Z

    @Guillaume: If you have also used "errors" inside the for-loop it would have worked. I'm sure the problem was that the for-loop variable was "errors", but inside the loop you used "error".

  • #149 David said 2018-08-07T16:20:26Z

    When I type in "flask run" into my terminal, this error appears:

    Error: While importing "microblog", an ImportError was raised:

    Traceback (most recent call last): File "/Users/jianrongtang/microblog/venv/lib/python3.7/site-packages/flask/cli.py", line 235, in locate_app import(module_name) File "/Users/jianrongtang/microblog/microblog.py", line 1, in from app import app, db File "/Users/jianrongtang/microblog/app/init.py", line 11, in from app import routes, models File "/Users/jianrongtang/microblog/app/routes.py", line 30, in from app.models import User File "/Users/jianrongtang/microblog/app/models.py", line 30, in from app import login ImportError: cannot import name 'login' from 'app' (/Users/jianrongtang/microblog/app/init.py)

    Can you help me diagnose it?

  • #150 Miguel Grinberg said 2018-08-07T21:20:49Z

    @David: the "app" package is missing a "login" symbol. Since the error refers to a package, look in the file init.py inside the package. You are probably missing the "login" object from Flask-Login there.

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