The difference between the two is that method one creates a connection on g.db whether you need it or not while method two only creates the connection when you call get_db for the first time in that application context.
If you compare the two, using this setup:
yourapp = Flask(__name__)
# setup g.db or app_context here
# Add a logging statement (print will do)
# to the get_db or before_request functions
# that simply says "Getting the db connection ..."
# Then access / and /1
@yourapp.route("/")
def index():
return "No database calls here!"
@yourapp.route("/")
def show_post(post_id):
# get a post using g.db or get_db
return "Went to the DB and got {!r}".format(post)
You'll see that when you hit / using the @app.before_request setup (g.db) you get a connection whether you use it or not, while using the _app_context route you only get a connection when you call get_db.
To be fair, you can also add a descriptor to g that will do the same lazy connecting (or in real life, acquiring a connection from a connection pool). And in both cases you can use a bit more magic (werkzeug.local.LocalProxy to be precise) to create your own custom thread local that acts like g, current_app and request (among others).
