Flask-Login stores a user id in the session.

On the next request, it needs to turn that id into a User object.

That’s what the user loader does.

Example

from flask_login import LoginManager
 
login_manager = LoginManager()
 
 
@login_manager.user_loader
def load_user(user_id: str):
    return User.query.get(int(user_id))

from flask_login import LoginManager
 
login_manager = LoginManager()
 
 
@login_manager.user_loader
def load_user(user_id: str):
    return User.query.get(int(user_id))

Important notes

• user_iduser_id comes from the session, so it’s a string.
• Convert types carefully.
• Return NoneNone if the user doesn’t exist.

Common pitfalls

• Forgetting to register the loader → Flask-Login can’t restore sessions
• Querying with the wrong type
• Returning something that isn’t a User model/UserMixin

Where to put this code

In larger apps, user loader registration usually lives in:

• authauth blueprint
• app factory initialization

So it’s registered exactly once.