Flask-Login#

Flask-Login provides user session management for Flask. It handles the common tasks of logging in, logging out, and remembering your users’ sessions over extended periods of time.

It will:

  • Store the active user’s ID in the Flask Session, and let you easily log them in and out.

  • Let you restrict views to logged-in (or logged-out) users. (login_required)

  • Handle the normally-tricky “remember me” functionality.

  • Help protect your users’ sessions from being stolen by cookie thieves.

However, it does not:

  • Impose a particular database or other storage method on you. You are entirely in charge of how the user is loaded.

  • Restrict you to using usernames and passwords, OpenIDs, or any other method of authenticating.

  • Handle permissions beyond “logged in or not.”

  • Handle user registration or account recovery.

Installation#

Install the extension with pip:

$ pip install flask-login

Configuring your Application#

The most important part of an application that uses Flask-Login is the LoginManager class. You should create one for your application somewhere in your code, like this:

from flask_login import LoginManager
login_manager = LoginManager()

The login manager contains the code that lets your application and Flask-Login work together, such as how to load a user from an ID, where to send users when they need to log in, and the like.

Once the actual application object has been created, you can configure it for login with:

login_manager.init_app(app)

By default, Flask-Login uses sessions for authentication. This means you must set the secret key on your application, otherwise Flask will give you an error message telling you to do so. See the Flask documentation on sessions to see how to set a secret key.

Warning: Make SURE to use the given command in the “How to generate good secret keys” section to generate your own secret key. DO NOT use the example one.

For a complete understanding of available configuration keys, please refer to the source code.

How it Works#

You will need to provide a user_loader callback. This callback is used to reload the user object from the user ID stored in the session. It should take the str ID of a user, and return the corresponding user object. For example:

@login_manager.user_loader
def load_user(user_id):
    return User.get(user_id)

It should return None (not raise an exception) if the ID is not valid. (In that case, the ID will manually be removed from the session and processing will continue.)

Your User Class#

The class that you use to represent users needs to implement these properties and methods:

is_authenticated

This property should return True if the user is authenticated, i.e. they have provided valid credentials. (Only authenticated users will fulfill the criteria of login_required.)

is_active

This property should return True if this is an active user - in addition to being authenticated, they also have activated their account, not been suspended, or any condition your application has for rejecting an account. Inactive accounts may not log in (without being forced of course).

is_anonymous

This property should return True if this is an anonymous user. (Actual users should return False instead.)

get_id()

This method must return a str that uniquely identifies this user, and can be used to load the user from the user_loader callback. Note that this must be a str - if the ID is natively an int or some other type, you will need to convert it to str.

To make implementing a user class easier, you can inherit from UserMixin, which provides default implementations for all of these properties and methods. (It’s not required, though.)

Login Example#

Once a user has authenticated, you log them in with the login_user function.

For example:

@app.route('/login', methods=['GET', 'POST'])
def login():
    # Here we use a class of some kind to represent and validate our
    # client-side form data. For example, WTForms is a library that will
    # handle this for us, and we use a custom LoginForm to validate.
    form = LoginForm()
    if form.validate_on_submit():
        # Login and validate the user.
        # user should be an instance of your `User` class
        login_user(user)

        flask.flash('Logged in successfully.')

        next = flask.request.args.get('next')
        # url_has_allowed_host_and_scheme should check if the url is safe
        # for redirects, meaning it matches the request host.
        # See Django's url_has_allowed_host_and_scheme for an example.
        if not url_has_allowed_host_and_scheme(next, request.host):
            return flask.abort(400)

        return flask.redirect(next or flask.url_for('index'))
    return flask.render_template('login.html', form=form)

Warning: You MUST validate the value of the next parameter. If you do not, your application will be vulnerable to open redirects. For an example implementation of url_has_allowed_host_and_scheme, see Django’s Implementation.

It’s that simple. You can then access the logged-in user with the current_user proxy, which is available in every template:

{% if current_user.is_authenticated %}
  Hi {{ current_user.name }}!
{% endif %}

Views that require your users to be logged in can be decorated with the login_required decorator:

@app.route("/settings")
@login_required
def settings():
    pass

When the user is ready to log out:

@app.route("/logout")
@login_required
def logout():
    logout_user()
    return redirect(somewhere)

They will be logged out, and any cookies for their session will be cleaned up.

Customizing the Login Process#

By default, when a user attempts to access a login_required view without being logged in, Flask-Login will flash a message and redirect them to the log in view. (If the login view is not set, it will abort with a 401 error.)

The name of the log in view can be set as LoginManager.login_view. For example:

login_manager.login_view = "users.login"

The default message flashed is Please log in to access this page. To customize the message, set LoginManager.login_message:

login_manager.login_message = u"Bonvolu ensaluti por uzi tiun paĝon."

To customize the message category, set LoginManager.login_message_category:

login_manager.login_message_category = "info"

When the log in view is redirected to, it will have a next variable in the query string, which is the page that the user was trying to access. Alternatively, if USE_SESSION_FOR_NEXT is True, the page is stored in the session under the key next.

If you would like to customize the process further, decorate a function with LoginManager.unauthorized_handler:

@login_manager.unauthorized_handler
def unauthorized():
    # do stuff
    return a_response

For example: You are using Flask Login with Flask Restful. In your API (blueprint named as api) you don’t wanna redirect to login page but return Unauthorized status code .:

from flask import redirect, url_for, request
from http import HTTPStatus
@login_manager.unauthorized_handler
def unauthorized():
    if request.blueprint == 'api':
        abort(HTTPStatus.UNAUTHORIZED)
    return redirect(url_for('site.login'))

Custom Login using Request Loader#

Sometimes you want to login users without using cookies, such as using header values or an api key passed as a query argument. In these cases, you should use the request_loader callback. This callback should behave the same as your user_loader callback, except that it accepts the Flask request instead of a user_id.

For example, to support login from both a url argument and from Basic Auth using the Authorization header:

@login_manager.request_loader
def load_user_from_request(request):

    # first, try to login using the api_key url arg
    api_key = request.args.get('api_key')
    if api_key:
        user = User.query.filter_by(api_key=api_key).first()
        if user:
            return user

    # next, try to login using Basic Auth
    api_key = request.headers.get('Authorization')
    if api_key:
        api_key = api_key.replace('Basic ', '', 1)
        try:
            api_key = base64.b64decode(api_key)
        except TypeError:
            pass
        user = User.query.filter_by(api_key=api_key).first()
        if user:
            return user

    # finally, return None if both methods did not login the user
    return None

Anonymous Users#

By default, when a user is not actually logged in, current_user is set to an AnonymousUserMixin object. It has the following properties and methods:

If you have custom requirements for anonymous users (for example, they need to have a permissions field), you can provide a callable (either a class or factory function) that creates anonymous users to the LoginManager with:

login_manager.anonymous_user = MyAnonymousUser

Remember Me#

By default, when the user closes their browser the Flask Session is deleted and the user is logged out. “Remember Me” prevents the user from accidentally being logged out when they close their browser. This does NOT mean remembering or pre-filling the user’s username or password in a login form after the user has logged out.

“Remember Me” functionality can be tricky to implement. However, Flask-Login makes it nearly transparent - just pass remember=True to the login_user call. A cookie will be saved on the user’s computer, and then Flask-Login will automatically restore the user ID from that cookie if it is not in the session. The amount of time before the cookie expires can be set with the REMEMBER_COOKIE_DURATION configuration or it can be passed to login_user. The cookie is tamper-proof, so if the user tampers with it (i.e. inserts someone else’s user ID in place of their own), the cookie will merely be rejected, as if it was not there.

That level of functionality is handled automatically. However, you can (and should, if your application handles any kind of sensitive data) provide additional infrastructure to increase the security of your remember cookies.

Alternative Tokens#

Using the user ID as the value of the remember token means you must change the user’s ID to invalidate their login sessions. One way to improve this is to use an alternative user id instead of the user’s ID. For example:

@login_manager.user_loader
def load_user(user_id):
    return User.query.filter_by(alternative_id=user_id).first()

Then the get_id method of your User class would return the alternative id instead of the user’s primary ID:

def get_id(self):
    return str(self.alternative_id)

This way you are free to change the user’s alternative id to a new randomly generated value when the user changes their password, which would ensure their old authentication sessions will cease to be valid. Note that the alternative id must still uniquely identify the user… think of it as a second user ID.

Fresh Logins#

When a user logs in, their session is marked as “fresh,” which indicates that they actually authenticated on that session. When their session is destroyed and they are logged back in with a “remember me” cookie, it is marked as “non-fresh.” login_required does not differentiate between freshness, which is fine for most pages. However, sensitive actions like changing one’s personal information should require a fresh login. (Actions like changing one’s password should always require a password re-entry regardless.)

fresh_login_required, in addition to verifying that the user is logged in, will also ensure that their login is fresh. If not, it will send them to a page where they can re-enter their credentials. You can customize its behavior in the same ways as you can customize login_required, by setting LoginManager.refresh_view, needs_refresh_message, and needs_refresh_message_category:

login_manager.refresh_view = "accounts.reauthenticate"
login_manager.needs_refresh_message = (
    u"To protect your account, please reauthenticate to access this page."
)
login_manager.needs_refresh_message_category = "info"

Or by providing your own callback to handle refreshing:

@login_manager.needs_refresh_handler
def refresh():
    # do stuff
    return a_response

To mark a session as fresh again, call the confirm_login function.

Session Protection#

While the features above help secure your “Remember Me” token from cookie thieves, the session cookie is still vulnerable. Flask-Login includes session protection to help prevent your users’ sessions from being stolen.

You can configure session protection on the LoginManager, and in the app’s configuration. If it is enabled, it can operate in either basic or strong mode. To set it on the LoginManager, set the session_protection attribute to "basic" or "strong":

login_manager.session_protection = "strong"

Or, to disable it:

login_manager.session_protection = None

By default, it is activated in "basic" mode. It can be disabled in the app’s configuration by setting the SESSION_PROTECTION setting to None, "basic", or "strong".

When session protection is active, each request, it generates an identifier for the user’s computer (basically, a secure hash of the IP address and user agent). If the session does not have an associated identifier, the one generated will be stored. If it has an identifier, and it matches the one generated, then the request is OK.

If the identifiers do not match in basic mode, or when the session is permanent, then the session will simply be marked as non-fresh, and anything requiring a fresh login will force the user to re-authenticate. (Of course, you must be already using fresh logins where appropriate for this to have an effect.)

If the identifiers do not match in strong mode for a non-permanent session, then the entire session (as well as the remember token if it exists) is deleted.

Automated Testing#

To make it easier for you to write automated tests, Flask-Login provides a simple, custom test client class that will set the user’s login cookie for you: FlaskLoginClient. To use this custom test client class, assign it to the test_client_class attribute on your application object, like this:

from flask_login import FlaskLoginClient

app.test_client_class = FlaskLoginClient

Next, use the app.test_client() method to make a test client, as you normally do. However, now you can pass a user object to this method, and your client will be automatically logged in with this user!

def test_request_with_logged_in_user():
    user = User.query.get(1)
    with app.test_client(user=user) as client:
        # This request has user 1 already logged in!
        client.get("/")

You may also pass fresh_login (bool, defaults to True) to mark the current login as fresh or non-fresh.

Note that you must use keyword arguments, not positional arguments. E.g. test_client(user=user) will work, but test_client(user) will not.

Due to the way this custom test client class is implemented, you may have to disable session protection to have your tests work properly. If session protection is enabled, login sessions will be marked non-fresh in basic mode or outright rejected in strong mode when performing requests with the test client.

Localization#

By default, the LoginManager uses flash to display messages when a user is required to log in. These messages are in English. If you require localization, set the localize_callback attribute of LoginManager to a function to be called with these messages before they’re sent to flash, e.g. gettext. This function will be called with the message and its return value will be sent to flash instead.

API Documentation#

This documentation is automatically generated from Flask-Login’s source code.

Configuring Login#

class flask_login.LoginManager(app=None, add_context_processor=True)#

This object is used to hold the settings used for logging in. Instances of LoginManager are not bound to specific apps, so you can create one in the main body of your code and then bind it to your app in a factory function.

init_app(app, add_context_processor=True)#

Configures an application. This registers an after_request call, and attaches this LoginManager to it as app.login_manager.

Parameters:

  • app (flask.Flask) – The flask.Flask object to configure.

  • add_context_processor (bool) – Whether to add a context processor to the app that adds a current_user variable to the template. Defaults to True.

unauthorized()#

This is called when the user is required to log in. If you register a callback with LoginManager.unauthorized_handler(), then it will be called. Otherwise, it will take the following actions:

  • Flash LoginManager.login_message to the user.

  • If the app is using blueprints find the login view for the current blueprint using blueprint_login_views. If the app is not using blueprints or the login view for the current blueprint is not specified use the value of login_view.

  • Redirect the user to the login view. (The page they were attempting to access will be passed in the next query string variable, so you can redirect there if present instead of the homepage. Alternatively, it will be added to the session as next if USE_SESSION_FOR_NEXT is set.)

If LoginManager.login_view is not defined, then it will simply raise a HTTP 401 (Unauthorized) error instead.

This should be returned from a view or before/after_request function, otherwise the redirect will have no effect.

needs_refresh()#

This is called when the user is logged in, but they need to be reauthenticated because their session is stale. If you register a callback with needs_refresh_handler, then it will be called. Otherwise, it will take the following actions:

If LoginManager.refresh_view is not defined, then it will simply raise a HTTP 401 (Unauthorized) error instead.

This should be returned from a view or before/after_request function, otherwise the redirect will have no effect.

General Configuration

user_loader(callback)#

This sets the callback for reloading a user from the session. The function you set should take a user ID (a str) and return a user object, or None if the user does not exist.

Parameters:

callback (callable) – The callback for retrieving a user object.

request_loader(callback)#

This sets the callback for loading a user from a Flask request. The function you set should take Flask request object and return a user object, or None if the user does not exist.

Parameters:

callback (callable) – The callback for retrieving a user object.

anonymous_user#

A class or factory function that produces an anonymous user, which is used when no one is logged in.

unauthorized Configuration

login_view#

The name of the view to redirect to when the user needs to log in. (This can be an absolute URL as well, if your authentication machinery is external to your application.)

blueprint_login_views#

This is similar to login_view, except it is used when working with blueprints. It is a dictionary that can store multiple views to redirect to for different blueprints. The redirects are listed in the form of key as the blueprint’s name and value as the redirect to route.

login_message#

The message to flash when a user is redirected to the login page.

unauthorized_handler(callback)#

This will set the callback for the unauthorized method, which among other things is used by login_required. It takes no arguments, and should return a response to be sent to the user instead of their normal view.

Parameters:

callback (callable) – The callback for unauthorized users.

needs_refresh Configuration

refresh_view#

The name of the view to redirect to when the user needs to reauthenticate.

needs_refresh_message#

The message to flash when a user is redirected to the reauthentication page.

needs_refresh_handler(callback)#

This will set the callback for the needs_refresh method, which among other things is used by fresh_login_required. It takes no arguments, and should return a response to be sent to the user instead of their normal view.

Parameters:

callback (callable) – The callback for unauthorized users.

Login Mechanisms#

flask_login.current_user#

A proxy for the current user.

flask_login.login_fresh()#

This returns True if the current login is fresh.

flask_login.login_remembered()#

This returns True if the current login is remembered across sessions.

flask_login.login_user(user, remember=False, duration=None, force=False, fresh=True)#

Logs a user in. You should pass the actual user object to this. If the user’s is_active property is False, they will not be logged in unless force is True.

This will return True if the log in attempt succeeds, and False if it fails (i.e. because the user is inactive).

Parameters:

  • user (object) – The user object to log in.

  • remember (bool) – Whether to remember the user after their session expires. Defaults to False.

  • duration (datetime.timedelta) – The amount of time before the remember cookie expires. If None the value set in the settings is used. Defaults to None.

  • force (bool) – If the user is inactive, setting this to True will log them in regardless. Defaults to False.

  • fresh (bool) – setting this to False will log in the user with a session marked as not “fresh”. Defaults to True.

flask_login.logout_user()#

Logs a user out. (You do not need to pass the actual user.) This will also clean up the remember me cookie if it exists.

flask_login.confirm_login()#

This sets the current session as fresh. Sessions become stale when they are reloaded from a cookie.

Protecting Views#

flask_login.login_required(func)#

If you decorate a view with this, it will ensure that the current user is logged in and authenticated before calling the actual view. (If they are not, it calls the LoginManager.unauthorized callback.) For example:

@app.route('/post')
@login_required
def post():
    pass

If there are only certain times you need to require that your user is logged in, you can do so with:

if not current_user.is_authenticated:
    return current_app.login_manager.unauthorized()

…which is essentially the code that this function adds to your views.

Note

Per W3 guidelines for CORS preflight requests, HTTP OPTIONS requests are exempt from login checks.

Parameters:

func (function) – The view function to decorate.

flask_login.fresh_login_required(func)#

If you decorate a view with this, it will ensure that the current user’s login is fresh - i.e. their session was not restored from a ‘remember me’ cookie. Sensitive operations, like changing a password or e-mail, should be protected with this, to impede the efforts of cookie thieves.

If the user is not authenticated, LoginManager.unauthorized() is called as normal. If they are authenticated, but their session is not fresh, it will call LoginManager.needs_refresh() instead. (In that case, you will need to provide a LoginManager.refresh_view.)

Behaves identically to the login_required() decorator with respect to configuration variables.

Note

Per W3 guidelines for CORS preflight requests, HTTP OPTIONS requests are exempt from login checks.

Parameters:

func (function) – The view function to decorate.

User Object Helpers#

class flask_login.UserMixin#

This provides default implementations for the methods that Flask-Login expects user objects to have.

class flask_login.AnonymousUserMixin#

This is the default object for representing an anonymous user.

Utilities#

flask_login.login_url(login_view, next_url=None, next_field='next')#

Creates a URL for redirecting to a login page. If only login_view is provided, this will just return the URL for it. If next_url is provided, however, this will append a next=URL parameter to the query string so that the login view can redirect back to that URL. Flask-Login’s default unauthorized handler uses this function when redirecting to your login url.

To force the host name used, set FORCE_HOST_FOR_REDIRECTS to a host. This prevents from redirecting to external sites if SERVER_NAME is not configured.

Parameters:

  • login_view (str) – The name of the login view. (Alternately, the actual URL to the login view.)

  • next_url (str) – The URL to give the login view for redirection.

  • next_field (str) – What field to store the next URL in. (It defaults to next.)

class flask_login.FlaskLoginClient(*args, **kwargs)#

A Flask test client that knows how to log in users using the Flask-Login extension.

Signals#

See the Flask documentation on signals for information on how to use these signals in your code.

flask_login.user_logged_in#

Sent when a user is logged in. In addition to the app (which is the sender), it is passed user, which is the user being logged in.

flask_login.user_logged_out#

Sent when a user is logged out. In addition to the app (which is the sender), it is passed user, which is the user being logged out.

flask_login.user_login_confirmed#

Sent when a user’s login is confirmed, marking it as fresh. (It is not called for a normal login.) It receives no additional arguments besides the app.

flask_login.user_unauthorized#

Sent when the unauthorized method is called on a LoginManager. It receives no additional arguments besides the app.

flask_login.user_needs_refresh#

Sent when the needs_refresh method is called on a LoginManager. It receives no additional arguments besides the app.

flask_login.session_protected#

Sent whenever session protection takes effect, and a session is either marked non-fresh or deleted. It receives no additional arguments besides the app.