flask/docs/tutorial/dbcon.rst

.. _tutorial-dbcon:

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.  So 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.  So 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`.

.. hint:: 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
   ``get_db`` and ``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.

.. _example source:
   https://github.com/mitsuhiko/flask/tree/master/examples/flaskr/