flask/docs/tutorial/setup.rst

.. _tutorial-setup:

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

Next, we will create the application module, :file:`flaskr.py`.  Just like the
:file:`schema.sql` file you created in the previous step, this file should be
placed inside of the :file:`flaskr/flaskr` folder.

For this tutorial, all the Python code we use will be put into this file
(except for one line in ``__init__.py``, and any testing or optional files you
decide to create).

The first several lines of code in the application module are the needed import
statements.  After that there will be a few lines of configuration code.

For small applications like ``flaskr``, it is possible to drop the configuration
directly into the module.  However, a cleaner solution is to create a separate
``.py`` file, load that, and import the values from there.

Here are the import statements (in :file:`flaskr.py`)::

    import os
    import sqlite3

    from flask import (Flask, request, session, g, redirect, url_for, abort,
        render_template, flash)

The next couple lines will create the actual application instance and
initialize it with the config from the same file in :file:`flaskr.py`::

    app = Flask(__name__) # create the application instance :)
    app.config.from_object(__name__) # load config from this file , flaskr.py

    # Load default config and override config from an environment variable
    app.config.update(
        DATABASE=os.path.join(app.root_path, 'flaskr.db'),
        SECRET_KEY=b'_5#y2L"F4Q8z\n\xec]/',
        USERNAME='admin',
        PASSWORD='default'
    )
    app.config.from_envvar('FLASKR_SETTINGS', silent=True)

In the above code, the :class:`~flask.Config` object works similarly to a
dictionary, so it can be updated with new values.

.. admonition:: Database Path

    Operating systems know the concept of a current working directory for
    each process.  Unfortunately, you cannot depend on this in web
    applications because you might have more than one application in the
    same process.

    For this reason the ``app.root_path`` attribute can be used to
    get the path to the application.  Together with the ``os.path`` module,
    files can then easily be found.  In this example, we place the
    database right next to it.

    For a real-world application, it's recommended to use
    :ref:`instance-folders` instead.

Usually, it is a good idea to load a separate, environment-specific
configuration file.  Flask allows you to import multiple configurations and it
will use the setting defined in the last import.  This enables robust
configuration setups.  :meth:`~flask.Config.from_envvar` can help achieve
this. ::

   app.config.from_envvar('FLASKR_SETTINGS', silent=True)

If you want to do this (not required for this tutorial) simply define the
environment variable :envvar:`FLASKR_SETTINGS` that points to a config file
to be loaded.  The silent switch just tells Flask to not complain if no such
environment key is set.

In addition to that, you can use the :meth:`~flask.Config.from_object`
method on the config object and provide it with an import name of a
module.  Flask will then initialize the variable from that module.  Note
that in all cases, only variable names that are uppercase are considered.

The :data:`SECRET_KEY` is needed to keep the client-side sessions secure.
Choose that key wisely and as hard to guess and complex as possible.

Lastly, add a method that allows for easy connections to the specified
database. ::

    def connect_db():
        """Connects to the specific database."""

        rv = sqlite3.connect(app.config['DATABASE'])
        rv.row_factory = sqlite3.Row
        return rv

This 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.
You can create a simple database connection through SQLite and then tell
it to use the :class:`sqlite3.Row` object to represent rows. This allows
the rows to be treated as if they were dictionaries instead of tuples.

In the next section you will see how to run the application.

Continue with :ref:`tutorial-packaging`.