python - How to use g.user global in flask

Question

As I understand the g variable in Flask, it should provide me with a global place to stash data like holding the current user after login. Is this correct?

I would like my navigation to display my user's name, once logged in, across the site.

My views contain

from Flask import g #among other things

During login, I assign

user = User.query.filter_by(username = form.username.data).first()
if validate(user):
    session['logged_in'] = True
    g.user = user

It doesn't seem I can access g.user. Instead, when my base.html template has the following...

<ul class="nav">
    {% if session['logged_in'] %}
        <li class="inactive">logged in as {{ g.user.username }}</li>
    {% endif %}
</ul>

I get the error:

jinja2.exceptions.UndefinedError
UndefinedError: 'flask.ctx._RequestGlobals object' has no attribute 'user'

The login otherwise works fine. What am I missing?

g.user isn't specific to a session- if I understand what you're trying to do correctly, you'd probably be better off doing something like session['user'] = user instead. — Nathan
Yikes! Okay, if it isn't specific to the session, it sounds like you're right. — Mittenchops
Wait, can that be true? I'm looking at the most extensive online tutorial I've found: blog.miguelgrinberg.com/post/… and he frequently uses the g variable for things like g.user = current_user. Is that actually incorrect? — Mittenchops
Actually, both you and @SeanVieira are right and I am thoroughly and helplessly misguided- even the Flask docs say that I'm wrong. Sorry for the trouble. :/ — Nathan

Sean Vieira Sean Vieira · Accepted Answer · 2012-11-29T17:40:32

g is a thread local and is per-request (See A Note On Proxies). The session is also a thread local, but in the default context is persisted to a MAC-signed cookie and sent to the client.

The problem that you are running into is that session is rebuilt on each request (since it is sent to the client and the client sends it back to us), while data set on g is only available for the lifetime of this request.

The simplest thing to do (note simple != secure - if you need secure take a look at Flask-Login) is to simply add the user's ID to the session and load the user on each request:

@app.before_request
def load_user():
    if session["user_id"]:
        user = User.query.filter_by(username=session["user_id"]).first()
    else:
        user = {"name": "Guest"}  # Make it better, use an anonymous User instead

    g.user = user

python - How to use g.user global in flask

3 Answers