Request setup, teardown, and application globals

In some cases, a request-specific variable is needed across all view functions and needs to be accessed from the template as well. To achieve this, we can use Flask's decorator function @app.before_request and the object g. The function @app.before_request is executed every time before a new request is made. The Flask object g is a thread-safe store of any data that needs to be kept for each specific request. At the end of the request, the object is destroyed, and a new object is spawned at the start of a new request. For example, the following code checks whether the Flask session variable contains an entry for a logged in user; if it exists, it adds the User object to g:

from flask import g, session, abort, render_template

@app.before_request
def before_request():
    if ‘user_id’ in session:
        g.user = User.query.get(session[‘user_id’])

@app.route(‘/restricted’)
def admin():
    if g.user is None:
        abort(403)
    return render_template...