flask-login: can't understand how it works

Flask-login doesn't actually have a user backend, it just handles the session machinery to help you login and logout users. You have to tell it (by decorating methods), what represents a user and it is also up to you to figure out how to know if a user is "active" or not (since being "active" can mean different things in different applications).

You should read the documentation and be sure what it does and does not do. Here I am only going to concentrate on wiring it up with the db backend.

To start off with, define a user object; which represents properties for your users. This object can then query databases, or LDAP, or whatever and it is the hook that connects the login mechanism with your database backend.

I will be using the login example script for this purpose.

class User(UserMixin):
    def __init__(self, name, id, active=True):
        self.name = name
        self.id = id
        self.active = active

    def is_active(self):
        # Here you should write whatever the code is
        # that checks the database if your user is active
        return self.active

    def is_anonymous(self):
        return False

    def is_authenticated(self):
        return True

Once you have the user object created, you need to write a method that loads the user (basically, creates an instance of the User class from above). This method is called with the user id.

@login_manager.user_loader
def load_user(id):
     # 1. Fetch against the database a user by `id` 
     # 2. Create a new object of `User` class and return it.
     u = DBUsers.query.get(id)
    return User(u.name,u.id,u.active)

Once you have these steps, your login method does this:

1. Checks to see if the username and password match (against your database) - you need to write this code yourself.

2. If authentication was successful you should pass an instance of the user to login_user()

Flask-login will try and load a user BEFORE every request. So yes, your example code below will be called before every request. It is used to check what userid is in the current session and will load the user object for that id.

@login_manager.user_loader
def load_user(userid):
    #print 'this is executed',userid
    return user(userid, 'asdf')        

If you look at the Flask-login source code on github, there is a line under function init_app which goes:

app.before_request(self._load_user)

So before every request, the _load_user function is called. The _load_user functions actually calls another function "reload_user()" based on conditions. And finally, reload_user() function calls your callback function that you wrote (load_user() in your example).

Also, flask-login only provides the mechanism to login/logout a user. It does not care if you are using mysql database.

As per from the Flask-Login's document a user object must be returned and if the user id is not found it should return None instead of Exception.

@login_manager.user_loader
def load_user(userid):
    try:
        #: Flask Peewee used here to return the user object
        return User.get(User.id==userid)
    except User.DoesNotExist:
        return None