Q:

Flask: setting application and request-specific attributes?

I am writing an application which connects to a database. I want to create that db connection once, and then reuse that connection throughout the life of the application.
I also want to authenticate users. A user's auth will live for only the life of a request.
How can I differentiate between objects stored for the life of a flask app, versus specific to the request? Where would I store them so that all modules (and subsequent blueprints) have access to them?
Here is my sample app:
from flask import Flask, g

app = Flask(__name__)

@app.before_first_request
def setup_database(*args, **kwargs):
    print 'before first request', g.__dict__
    g.database = 'DATABASE'
    print 'after first request', g.__dict__

@app.route('/')
def index():
    print 'request start', g.__dict__
    g.current_user = 'USER'
    print 'request end', g.__dict__

    return 'hello'

if __name__ == '__main__':
    app.run(debug=True, port=6001)

When I run this (Flask 0.10.1) and navigate to http://localhost:6001/, here is what shows up in the console:
$ python app.py 
 * Running on http://127.0.0.1:6001/
 * Restarting with reloader

before first request {}
after first request {'database': 'DATABASE'}
request start {'database': 'DATABASE'}
request end {'current_user': 'USER', 'database': 'DATABASE'}
127.0.0.1 - - [30/Sep/2013 11:36:40] "GET / HTTP/1.1" 200 -

request start {}
request end {'current_user': 'USER'}
127.0.0.1 - - [30/Sep/2013 11:36:41] "GET / HTTP/1.1" 200 -

That is, the first request is working as expected: flask.g is holding my database, and when the request starts, it also has my user's information.
However, upon my second request, flask.g is wiped clean! My database is nowhere to be found.
Now, I know that flask.g used to apply to the request only. But now that it is bound to the application (as of 0.10), I want to know how to bind variables to the entire application, rather than just a single request.
What am I missing?
edit: I'm specifically interested in MongoDB - and in my case, maintaining connections to multiple Mongo databases. Is my best bet to just create those connections in __init__.py and reuse those objects? 

A:

flask.g will only store things for the duration of a request. The documentation mentioned that the values are stored on the application context rather than the request, but that is more of an implementation issue: it doesn't change the fact that objects in flask.g are only available in the same thread, and during the lifetime of a single request.
For example, in the official tutorial section on database connections, the connection is made once at the beginning of the request, then terminated at the end of the request.
Of course, if you really wanted to, you could create the database connection once, store it in __init__.py, and reference it (as a global variable) as needed. However, you shouldn't do this: the connection could close or timeout, and you could not use the connection in multiple threads.
Since you didn't specify HOW you will be using Mongo in Python, I assume you will be using PyMongo, since that handles all of the connection pooling for you.
In this case, you would do something like this...
from flask import Flask
from pymongo import MongoClient
# This line of code does NOT create a connection
client = MongoClient()

app = Flask()

# This can be in __init__.py, or some other file that has imported the "client" attribute
@app.route('/'):
def index():
    posts = client.database.posts.find()

You could, if you wish, do something like this...
from flask import Flask, g
from pymongo import MongoClient
# This line of code does NOT create a connection
client = MongoClient()

app = Flask()

@app.before_request
def before_request():
    g.db = client.database

@app.route('/'):
def index():
    posts = g.db.posts.find()

This really isn't all that different, however it can be helpful for logic that you want to perform on every request (such as setting g.db to a specific database depending on the user that is logged in).
Finally, you can realize that most of the work of setting up PyMongo with Flask is probably done for you in Flask-PyMongo.
Your other question deals with how you keep track of stuff specific to the user that is logged in. Well, in this case, you DO need to store some data that sticks around with the connection. flask.g is cleared at the end of the reuquest, so that's no good.
What you want to use is sessions. This is a place where you can store values that is (with the default implementation) stored in a cookie on the user's browser. Since the cookie will be passed along with every request the user's browser makes to your web site, you will have available the data you put in the session.
Keep in mind, though, that the session is NOT stored on the server. It is turned into a string that is passed back and forth to the user. Therefore, you can't store things like DB connections onto it. You would instead store identifiers (like user IDs).
Making sure that user authentication works is VERY hard to get right. The security concerns that you need to make sure of are amazingly complex. I would strongly recommend using something like Flask-Login to handle this for you. You can still use the session for storing other items as needed, or you can let Flask-Login handle determining the user ID and store the values you need in the database and retrieving them from the database in every request.
So, in summary, there are a few different ways to do what you want to do. Each have their usages.

Globals are good for items that are thread-safe (such as the PyMongo's MongoClient).
flask.g can be used for storing data in the lifetime of a request. With SQLAlchemy-based flask apps, a common thing to do is to ensure that all changes happen at once, at the end of a request using an after_request method. Using flask.g for something like this is very helpful.
The Flask session can be used to store simple data (strings and numbers, not connection objects) that can be used on subsequent requests that come from the same user. This is entirely dependent on using cookies, so at any point the user could delete the cookie and everything in the "session" will be lost. Therefore, you probably want to store much of your data in databases, with the session used to identify the data that relates to the user in the session.

A:

"bound to the application" does not mean what you think it means. It means that g is bound to the currently running request. Quoth the docs:

Flask provides you with a special object that ensures it is only valid for the active request and that will return different values for each request.

It should be noted that Flask's tutorials specifically do not persist database objects, but that this is not normative for any application of substantial size. If you're really interested in diving down the rabbit hole, I suggest a database connection pooling tool. (such as this one, mentioned in the SO answer ref'd above)

