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Flask-Login

.. currentmodule:: 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 session, and let you log them in and out easily.
  • Let you restrict views to logged-in (or logged-out) users.
  • Handle the normally-tricky "remember me" functionality.
  • Help protect your users' sessions from being stolen by cookie thieves.
  • Possibly integrate with Flask-Principal or other authorization extensions later on.

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:

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)

How it Works

You will need to provide a ~LoginManager.user_loader callback. This callback is used to reload the user object from the user ID stored in the session. It should take the unicode 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 unicode that uniquely identifies this user, and can be used to load the user from the ~LoginManager.user_loader callback. Note that this must be a unicode - if the ID is natively an int or some other type, you will need to convert it to unicode.

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')
        # is_safe_url should check if the url is safe for redirects.
        # See http://flask.pocoo.org/snippets/62/ for an example.
        if not is_safe_url(next):
            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 is_safe_url see this Flask Snippet.

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

Login using Authorization header

Caution!

This method will be deprecated; use the ~LoginManager.request_loader below instead.

Sometimes you want to support Basic Auth login using the Authorization header, such as for api requests. To support login via header you will need to provide a ~LoginManager.header_loader callback. This callback should behave the same as your ~LoginManager.user_loader callback, except that it accepts a header value instead of a user id. For example:

@login_manager.header_loader
def load_user_from_header(header_val):
    header_val = header_val.replace('Basic ', '', 1)
    try:
        header_val = base64.b64decode(header_val)
    except TypeError:
        pass
    return User.query.filter_by(api_key=header_val).first()

By default the Authorization header's value is passed to your ~LoginManager.header_loader callback. You can change the header used with the AUTH_HEADER_NAME configuration.

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 ~LoginManager.request_loader callback. This callback should behave the same as your ~LoginManager.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:

  • is_active and is_authenticated are False
  • is_anonymous is True
  • get_id() returns None

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 ~UserMixin.get_id method of your User class would return the alternative id instead of the user's primary ID:

def get_id(self):
    return unicode(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, ~LoginManager.needs_refresh_message, and ~LoginManager.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.

Cookie Settings

The details of the cookie can be customized in the application settings.

REMEMBER_COOKIE_NAME The name of the cookie to store the "remember me" information in. Default: remember_token
REMEMBER_COOKIE_DURATION The amount of time before the cookie expires, as a datetime.timedelta object or integer seconds. Default: 365 days (1 non-leap Gregorian year)
REMEMBER_COOKIE_DOMAIN If the "Remember Me" cookie should cross domains, set the domain value here (i.e. .example.com would allow the cookie to be used on all subdomains of example.com). Default: None
REMEMBER_COOKIE_PATH Limits the "Remember Me" cookie to a certain path. Default: /
REMEMBER_COOKIE_SECURE Restricts the "Remember Me" cookie's scope to secure channels (typically HTTPS). Default: None
REMEMBER_COOKIE_HTTPONLY Prevents the "Remember Me" cookie from being accessed by client-side scripts. Default: False
REMEMBER_COOKIE_REFRESH_EACH_REQUEST If set to True the cookie is refreshed on every request, which bumps the lifetime. Works like Flask's SESSION_REFRESH_EACH_REQUEST. Default: False

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 ~LoginManager.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.

Disabling Session Cookie for APIs

When authenticating to APIs, you might want to disable setting the Flask Session cookie. To do this, use a custom session interface that skips saving the session depending on a flag you set on the request. For example:

from flask import g
from flask.sessions import SecureCookieSessionInterface
from flask_login import user_loaded_from_header

class CustomSessionInterface(SecureCookieSessionInterface):
    """Prevent creating session from API requests."""
    def save_session(self, *args, **kwargs):
        if g.get('login_via_header'):
            return
        return super(CustomSessionInterface, self).save_session(*args,
                                                                **kwargs)

app.session_interface = CustomSessionInterface()

@user_loaded_from_header.connect
def user_loaded_from_header(self, user=None):
    g.login_via_header = True

This prevents setting the Flask Session cookie whenever the user authenticated using your ~LoginManager.header_loader.

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

.. module:: flask_login

.. autoclass:: LoginManager

   .. automethod:: setup_app

   .. automethod:: unauthorized

   .. automethod:: needs_refresh

   .. rubric:: General Configuration

   .. automethod:: user_loader

   .. automethod:: header_loader

   .. attribute:: anonymous_user

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

   .. rubric:: `unauthorized` Configuration

   .. attribute:: 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.)

   .. attribute:: login_message

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

   .. automethod:: unauthorized_handler

   .. rubric:: `needs_refresh` Configuration

   .. attribute:: refresh_view

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

   .. attribute:: needs_refresh_message

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

   .. automethod:: needs_refresh_handler


Login Mechanisms

.. data:: current_user

   A proxy for the current user.

.. autofunction:: login_fresh

.. autofunction:: login_user

.. autofunction:: logout_user

.. autofunction:: confirm_login


Protecting Views

.. autofunction:: login_required

.. autofunction:: fresh_login_required


User Object Helpers

.. autoclass:: UserMixin
   :members:

.. autoclass:: AnonymousUserMixin
   :members:


Utilities

.. autofunction:: login_url


Signals

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

.. data:: 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.

.. data:: 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.

.. data:: 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.

.. data:: user_unauthorized

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

.. data:: user_needs_refresh

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

.. data:: 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.