.. currentmodule:: flask

Application Setup
=================

A Flask application is an instance of the :class:`Flask` class.
Everything about the application, such as configuration and URLs, will
be registered with this class.

The most straightforward way to create a Flask application is to create
a global :class:`Flask` instance directly at the top of your code, like
how the "Hello, World!" example did on the previous page. While this is
simple and useful in some cases, it can cause some tricky issues as the
project grows.

Instead of creating a :class:`Flask` instance globally, you will create
it inside a function. This function is known as the *application
factory*. Any configuration, registration, and other setup the
application needs will happen inside the function, then the application
will be returned.


The Application Factory
-----------------------

It's time to start coding! Create the ``flaskr`` directory and add the
``__init__.py`` file. The ``__init__.py`` serves double duty: it will
contain the application factory, and it tells Python that the ``flaskr``
directory should be treated as a package.

.. code-block:: none

    $ mkdir flaskr

.. code-block:: python
    :caption: ``flaskr/__init__.py``

    import os

    from flask import Flask


    def create_app(test_config=None):
        # create and configure the app
        app = Flask(__name__, instance_relative_config=True)
        app.config.from_mapping(
            SECRET_KEY='dev',
            DATABASE=os.path.join(app.instance_path, 'flaskr.sqlite'),
        )

        if test_config is None:
            # load the instance config, if it exists, when not testing
            app.config.from_pyfile('config.py', silent=True)
        else:
            # load the test config if passed in
            app.config.from_mapping(test_config)

        # ensure the instance folder exists
        os.makedirs(app.instance_path, exist_ok=True)

        # a simple page that says hello
        @app.route('/hello')
        def hello():
            return 'Hello, World!'

        return app

``create_app`` is the application factory function. You'll add to it
later in the tutorial, but it already does a lot.

#.  ``app = Flask(__name__, instance_relative_config=True)`` creates the
    :class:`Flask` instance.

    *   ``__name__`` is the name of the current Python module. The app
        needs to know where it's located to set up some paths, and
        ``__name__`` is a convenient way to tell it that.

    *   ``instance_relative_config=True`` tells the app that
        configuration files are relative to the
        :ref:`instance folder <instance-folders>`. The instance folder
        is located outside the ``flaskr`` package and can hold local
        data that shouldn't be committed to version control, such as
        configuration secrets and the database file.

#.  :meth:`app.config.from_mapping() <Config.from_mapping>` sets
    some default configuration that the app will use:

    *   :data:`SECRET_KEY` is used by Flask and extensions to keep data
        safe. It's set to ``'dev'`` to provide a convenient value
        during development, but it should be overridden with a random
        value when deploying.

    *   ``DATABASE`` is the path where the SQLite database file will be
        saved. It's under
        :attr:`app.instance_path <Flask.instance_path>`, which is the
        path that Flask has chosen for the instance folder. You'll learn
        more about the database in the next section.

#.  :meth:`app.config.from_pyfile() <Config.from_pyfile>` overrides
    the default configuration with values taken from the ``config.py``
    file in the instance folder if it exists. For example, when
    deploying, this can be used to set a real ``SECRET_KEY``.

    *   ``test_config`` can also be passed to the factory, and will be
        used instead of the instance configuration. This is so the tests
        you'll write later in the tutorial can be configured
        independently of any development values you have configured.

#.  :func:`os.makedirs` ensures that
    :attr:`app.instance_path <Flask.instance_path>` exists. Flask
    doesn't create the instance folder automatically, but it needs to be
    created because your project will create the SQLite database file
    there.

#.  :meth:`@app.route() <Flask.route>` creates a simple route so you can
    see the application working before getting into the rest of the
    tutorial. It creates a connection between the URL ``/hello`` and a
    function that returns a response, the string ``'Hello, World!'`` in
    this case.


Run The Application
-------------------

Now you can run your application using the ``flask`` command. From the
terminal, tell Flask where to find your application, then run it in
debug mode. Remember, you should still be in the top-level
``flask-tutorial`` directory, not the ``flaskr`` package.

Debug mode shows an interactive debugger whenever a page raises an
exception, and restarts the server whenever you make changes to the
code. You can leave it running and just reload the browser page as you
follow the tutorial.

.. code-block:: text

    $ flask --app flaskr run --debug

You'll see output similar to this:

.. code-block:: text

     * Serving Flask app "flaskr"
     * Debug mode: on
     * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
     * Restarting with stat
     * Debugger is active!
     * Debugger PIN: nnn-nnn-nnn

Visit http://127.0.0.1:5000/hello in a browser and you should see the
"Hello, World!" message. Congratulations, you're now running your Flask
web application!

If another program is already using port 5000, you'll see
``OSError: [Errno 98]`` or ``OSError: [WinError 10013]`` when the
server tries to start. See :ref:`address-already-in-use` for how to
handle that.

Continue to :doc:`database`.
