flask/docs/patterns/packages.rst

.. _larger-applications:

Larger Applications
===================

For larger applications it's a good idea to use a package instead of a
module.  That is quite simple.  Imagine a small application looks like
this::

    /yourapplication
        /yourapplication.py
        /static
            /style.css
        /templates
            layout.html
            index.html
            login.html
            ...

To convert that into a larger one, just create a new folder
`yourapplication` inside the existing one and move everything below it.
Then rename `yourapplication.py` to `__init__.py`.  (Make sure to delete
all `.pyc` files first, otherwise things would most likely break)

You should then end up with something like that::

    /yourapplication
        /yourapplication
            /__init__.py
            /static
                /style.css
            /templates
                layout.html
                index.html
                login.html
                ...

But how do you run your application now?  The naive ``python
yourapplication/__init__.py`` will not work.  Let's just say that Python
does not want modules in packages to be the startup file.  But that is not
a big problem, just add a new file called `runserver.py` next to the inner
`yourapplication` folder with the following contents::

    from yourapplication import app
    app.run(debug=True)

What did we gain from this?  Now we can restructure the application a bit
into multiple modules.  The only thing you have to remember is the
following quick checklist:

1. the `Flask` application object creation have to be in the
   `__init__.py` file.  That way each module can import it safely and the
   `__name__` variable will resole to the correct package.
2. all the view functions (the ones with a :meth:`~flask.Flask.route`
   decorator on top) have to be imported when in the `__init__.py` file.
   Not the objects itself, but the module it is in.  Do the importing at
   the *bottom* of the file.

Here an example `__init__.py`::

    from flask import Flask
    app = Flask(__name__)

    import yourapplication.views

And this is what `views.py` would look like::

    from yourapplication import app

    @app.route('/')
    def index():
        return 'Hello World!'

.. admonition:: Circular Imports

   Every Python programmer hates them, and yet we just added some:
   circular imports (That's when two module depend on each one.  In this
   case `views.py` depends on `__init__.py`).  Be advised that this is a
   bad idea in general but here it is actually fine.  The reason for this
   is
   that we are not actually using the views in `__init__.py` and just
   ensuring the module is imported and we are doing that at the bottom of
   the file.

   There are still some problems with that approach but if you want to use
   decorators there is no way around that.  Check out the
   :ref:`becomingbig` section for some inspiration how to deal with that.