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.