testbed/pallets__flask/docs/patterns/packages.rst

Large Applications as Packages
==============================

Imagine a simple flask application structure that looks like this::

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

While this is fine for small applications, for larger applications
it's a good idea to use a package instead of a module.
The :doc:`/tutorial/index` is structured to use the package pattern,
see the :gh:`example code <examples/tutorial>`.

Simple Packages
---------------

To convert that into a larger one, just create a new folder
:file:`yourapplication` inside the existing one and move everything below it.
Then rename :file:`yourapplication.py` to :file:`__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 :file:`setup.py` next to the inner
:file:`yourapplication` folder with the following contents::

    from setuptools import setup

    setup(
        name='yourapplication',
        packages=['yourapplication'],
        include_package_data=True,
        install_requires=[
            'flask',
        ],
    )

In order to run the application you need to export an environment variable
that tells Flask where to find the application instance:

.. tabs::

   .. group-tab:: Bash

      .. code-block:: text

         $ export FLASK_APP=yourapplication

   .. group-tab:: Fish

      .. code-block:: text

         $ set -x FLASK_APP yourapplication

   .. group-tab:: CMD

      .. code-block:: text

         > set FLASK_APP=yourapplication

   .. group-tab:: Powershell

      .. code-block:: text

         > $env:FLASK_APP = "yourapplication"

If you are outside of the project directory make sure to provide the exact
path to your application directory. Similarly you can turn on the
development features like this:

.. tabs::

   .. group-tab:: Bash

      .. code-block:: text

         $ export FLASK_ENV=development

   .. group-tab:: Fish

      .. code-block:: text

         $ set -x FLASK_ENV development

   .. group-tab:: CMD

      .. code-block:: text

         > set FLASK_ENV=development

   .. group-tab:: Powershell

      .. code-block:: text

         > $env:FLASK_ENV = "development"

In order to install and run the application you need to issue the following
commands::

    $ pip install -e .
    $ flask run

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 has to be in the
   :file:`__init__.py` file.  That way each module can import it safely and the
   `__name__` variable will resolve to the correct package.
2. all the view functions (the ones with a :meth:`~flask.Flask.route`
   decorator on top) have to be imported in the :file:`__init__.py` file.
   Not the object itself, but the module it is in. Import the view module
   **after the application object is created**.

Here's an example :file:`__init__.py`::

    from flask import Flask
    app = Flask(__name__)

    import yourapplication.views

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

    from yourapplication import app

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

You should then end up with something like that::

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

.. admonition:: Circular Imports

   Every Python programmer hates them, and yet we just added some:
   circular imports (That's when two modules depend on each other.  In this
   case :file:`views.py` depends on :file:`__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 :file:`__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
   :doc:`/becomingbig` section for some inspiration how to deal with that.


Working with Blueprints
-----------------------

If you have larger applications it's recommended to divide them into
smaller groups where each group is implemented with the help of a
blueprint.  For a gentle introduction into this topic refer to the
:doc:`/blueprints` chapter of the documentation.