Assign a variable into `g` once and only once for application in Flask

Question

I want to save an object which is the result of a expensive function. The expensive function should only be processed once before any request.

I checked the document of Flask and considered about g for saving the result and @app.before_first_request decorator to define this assignment happended only once.

My codes are like this:

@app.before_first_request
def before_first_request():
    g.rec = take_long_time_to_do()

@app.route('/test/')
def test():
    return render_template('index.html',var_rec=g.rec)

However, these codes won't work well. It works only in the first time test request is called. When I access "myapplication/test" second time, the g.rec doesn't exist, which will throw an exception

Does anyone have ideas about how to assign a global variable into g when initing the application?

Answer 1

g is the global object for that request only. Have you considered using a caching mechanism?

> pip search flask | grep "cache" | sort
Flask-Cache               - Adds cache support to your Flask application
Flask-Cache-PyLibMC       - PyLibMC cache for Flask-Cache, supports multiple operations
Flask-Memsessions         - A simple extension to drop in memcached session support for Flask.

Then you can store the result of take_long_time_to_do() there and retrieve it if it exists.