UndefinedError : 'user' is undefined

Question

I am currently developing a Flask app (have been for the past year) and I'm encountering a rather... Weird bug. I've got a few files that are always included in my Jinja2 templates (navbars), and they use the users' name and avatar. As a consequence, everytime I render a template, I pass it the user. I recently noticed an error on my prod server :

<img alt="image" class="img-circle" src="{{ user.image }}" style="width: 48px;"/>
  File "/usr/local/lib/python2.7/dist-packages/jinja2/environment.py", line 397, in getattr
    return getattr(obj, attribute)
jinja2.exceptions.UndefinedError: 'user' is undefined

This is in one of my navbars. The method that renders this template uses this :

@mod.route('/broken_pus', methods=['POST', 'GET'])
def view_broken_pus():
    return render_template("view_broken_pus.html", user=g.user, urls_for_active_clients=DeletedURLs.objects()[0].urls_for_active_clients, other_urls=DeletedURLs.objects()[0].other_urls)

As you can see, I pass the user=g.user. I do this on every single view of my website. And it works everywhere, EXCEPT on this method, which is pretty small. I have plenty of other routes like that, with just a render template, so I don't get what's the problem.

I also get it on another method, bigger, which always worked before :

@mod.route('/users/add', methods=['GET', 'POST'])
@requires_roles("admin", "project-leader")
def add():
    """
    Method adding a new user.
    """
    # We do not use WTForms there since we need custom checkboxes for the role
    # Instead we use basic HTML and treat the checkboxes here
    if request.method == 'POST':
        user = User(name=request.form.get('name'),
                    email=request.form.get('email'))
        l = []
        # big switch assignement
        user.role = l
        try:
            user.save()
        except errors.NotUniqueError:
            flash(u'User %s already in database.' % user.name, 'danger')
            return redirect(url_for('home'))
        flash(u'User %s registered.' % user.name, 'success')
        return redirect(url_for('home'))
    return render_template('add_user.html', page=url_for('users.add'), user=g.user, clients=Client.objects())

When I first load the form for adding a user, it works. When I add it, for some reason, I get the error (and the user is not saved in the database).

Since this works perfectly on local, I'm starting to suspect a problem on the production server itself. We use nginx and uwsgi for the app, and I recently implemented some Celery tasks. Got any idea ?

Thanks in advance.

Answer 1

Check out flask source for render_template:

It just calls template.render(context), but after the call to before_render_template.send(app, template=template, context=context)

From this, I think there is some before_render_template handler, that modifies context installed.

To debug this down, I may try to call something like this:

from flask import app

@mod.route('/broken_pus', methods=['POST', 'GET'])
def view_broken_pus():
    template = app.jinja_env.get_or_select_template("view_broken_pus.html")
    return template.render(dict(
        user=g.user,
        urls_for_active_clients=DeletedURLs.objects()[0].urls_for_active_clients,
        other_urls=DeletedURLs.objects()[0].other_urls,
    ))

If this will work, I will need to dig in who modifies context in before_render_template slot.

Answer 2

I suspect threading. If g is some sort of global reference then you may need to ensure that it is set up on threading.local or that threading locks are used to ensure that no thread can get hold of g.user before some 'other' thread messes with it.

See how do I make a 2.7 python context manager threadsafe for a way to handle 'globals' without sacrificing thread safety.