Globals

As discussed earlier in this chapter, Flask provides a mechanism to store global variables that are unique to a particular thread and request context. That's used for request and session, but is also available to store any custom object.

The flask.g variable contains all globals, and you can set whatever attributes you want on it.

In Flask, the @app.before_request decorator can be used to point a function that the app will call every time a request is made just before it dispatches the request to a view.

It's a typical pattern in Flask to use before_request to set values in the globals. That way, all the functions that are called within the request context can interact with g and get the data.

In the following example, we copy the username provided when the client performs an HTTP basic authentication in the user attribute:

    from flask import Flask, jsonify, g, request 
 
    app = Flask(__name__) 
 
    @app.before_request 
    def authenticate(): 
        if request.authorization: 
            g.user = request.authorization['username'] 
        else: 
            g.user = 'Anonymous' 
 
    @app.route('/api') 
    def my_microservice(): 
        return jsonify({'Hello': g.user}) 
 
    if __name__ == '__main__': 
        app.run() 

When a client requests the /api view, the authenticate function will set g.user depending on the provided headers:

$ curl http://127.0.0.1:5000/api 
{ 
  "Hello": "Anonymous" 
} 
$ curl http://127.0.0.1:5000/api --user tarek:pass 
{ 
  "Hello": "tarek" 
} 

Any data you may think of that's specific to a request context, and could be shared throughout your code, can be shared via flask.g.