Step 3: Database Connections
We have created a function for establishing a database connection with connect_db, but by itself, that's not particularly useful. Creating and closing database connections all the time is very inefficient, so we want to keep it around for longer. Because database connections encapsulate a transaction, we also need to make sure that only one request at the time uses the connection. How can we elegantly do that with Flask?
This is where the application context comes into play, so let's start there.
Flask provides us with two contexts: the application context and the request context. For the time being, all you have to know is that there are special variables that use these. For instance, the :data:`~flask.request` variable is the request object associated with the current request, whereas :data:`~flask.g` is a general purpose variable associated with the current application context. We will go into the details of this a bit later.
For the time being, all you have to know is that you can store information safely on the :data:`~flask.g` object.
So when do you put it on there? To do that you can make a helper function. The first time the function is called, it will create a database connection for the current context, and successive calls will return the already established connection:
def get_db(): """Opens a new database connection if there is none yet for the current application context. """ if not hasattr(g, 'sqlite_db'): g.sqlite_db = connect_db() return g.sqlite_db
So now we know how to connect, but how do we properly disconnect? For that, Flask provides us with the :meth:`~flask.Flask.teardown_appcontext` decorator. It's executed every time the application context tears down:
@app.teardown_appcontext def close_db(error): """Closes the database again at the end of the request.""" if hasattr(g, 'sqlite_db'): g.sqlite_db.close()
Functions marked with :meth:`~flask.Flask.teardown_appcontext` are called
every time the app context tears down. What does this mean?
Essentially, the app context is created before the request comes in and is
destroyed (torn down) whenever the request finishes. A teardown can
happen because of two reasons: either everything went well (the error
parameter will be
None) or an exception happened, in which case the error
is passed to the teardown function.
Curious about what these contexts mean? Have a look at the :ref:`app-context` documentation to learn more.
Continue to :ref:`tutorial-dbinit`.
Where do I put this code?
If you've been following along in this tutorial, you might be wondering
where to put the code from this step and the next. A logical place is to
group these module-level functions together, and put your new
close_db functions below your existing
connect_db function (following the tutorial line-by-line).
If you need a moment to find your bearings, take a look at how the example source is organized. In Flask, you can put all of your application code into a single Python module. You don't have to, and if your app :ref:`grows larger <larger-applications>`, it's a good idea not to.