flask/docs/tutorial/setup.rst

Step 2: Application Setup Code
==============================

Now that we have the schema in place we can create the application module.
Let's call it `flaskr.py` inside the `flaskr` folder.  For starters we
will add the imports we will need as well as the config section.  For
small applications it's a possibility to drop the configuration directly
into the module which we will be doing here.  However a cleaner solution
would be to create a separate `.ini` or `.py` file and load that or import
the values from there.

::

    # all the imports
    import sqlite3
    from flask import Flask, request, session, g, redirect, url_for, \
         abort, render_template, flash

    # configuration
    DATABASE = '/tmp/flaskr.db'
    DEBUG = True
    SECRET_KEY = 'development key'
    USERNAME = 'admin'
    PASSWORD = 'default'

Next we can create our actual application and initialize it with the
config::

    # create our little application :)
    app = Flask(__name__)
    app.secret_key = SECRET_KEY
    app.debug = DEBUG

The `secret_key` is needed to keep the client-side sessions secure.
Choose that key wisely and as hard to guess and complex as possible.  The
debug flag enables or disables the interactive debugger.  Never leave
debug mode activated in a production system because it will allow users to
executed code on the server!

We also add a method to easily connect to the database specified.  That
can be used to open a connection on request and also from the interactive
Python shell or a script.  This will come in handy later

::

    def connect_db():
        return sqlite3.connect(DATABASE)

Finally we just add a line to the bottom of the file that fires up the
server if we run that file as standalone application::

    if __name__ == '__main__':
        app.run()

With that out of the way you should be able to start up the application
without problems.  When you head over to the server you will get an 404
page not found error because we don't have any views yet.  But we will
focus on that a little later.  First we should get the database working.

.. admonition:: Troubleshooting

   If you notice later that the browser cannot connect to the server
   during development, you might want to try this line instead::

       app.run(host='127.0.0.1')

   In a nutshell: Werkzeug starts up as IPv6 on many operating systems by
   default and not every browser is happy with that.  This forces IPv4
   usage.