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
- Configuring your Application
- How it Works
- Your User Class
- Login Example
- Customizing the Login Process
- Login using Authorization header
- Custom Login using Request Loader
- Anonymous Users
- Remember Me
- Alternative Tokens
- Fresh Logins
- Cookie Settings
- Session Protection
- Disabling Session Cookie for APIs
- Automated Testing
- Localization
- API Documentation
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)
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.
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 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 oflogin_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 returnFalse
instead.) get_id()
- This method must return a
unicode
that uniquely identifies this user, and can be used to load the user from theuser_loader
callback. Note that this must be aunicode
- if the ID is natively anint
or some other type, you will need to convert it tounicode
.
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 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 header_loader
callback. This callback should behave
the same as your 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
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 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 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
, 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.
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
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 header_loader
.
Automated Testing¶
To make it easier for you to write automated tests, Flask-Login provides a
custom test client class that will set the user’s login cookie for you.
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_simple(self):
user = User.query.get(1)
with app.test_client(user=user) as client:
# this request has user 1 already logged in!
resp = client.get("/")
Note that you must use a keyword argument, not a positional argument.
test_client(user=user)
will work, but test_client(user)
will not.
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¶
Protecting Views¶
User Object Helpers¶
Utilities¶
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.
Sent when the
unauthorized
method is called on aLoginManager
. It receives no additional arguments besides the app.
-
flask_login.
user_needs_refresh
¶ Sent when the
needs_refresh
method is called on aLoginManager
. 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.