Flask Extension Development
===========================

Flask, being a microframework, often requires some repetitive steps to get
a third party library working. Many such extensions are already available
on `PyPI`_.

If you want to create your own Flask extension for something that does not
exist yet, this guide to extension development will help you get your
extension running in no time and to feel like users would expect your
extension to behave.

Anatomy of an Extension
-----------------------

Extensions are all located in a package called ``flask_something``
where "something" is the name of the library you want to bridge.  So for
example if you plan to add support for a library named `simplexml` to
Flask, you would name your extension's package ``flask_simplexml``.

The name of the actual extension (the human readable name) however would
be something like "Flask-SimpleXML".  Make sure to include the name
"Flask" somewhere in that name and that you check the capitalization.
This is how users can then register dependencies to your extension in
their :file:`setup.py` files.

But what do extensions look like themselves?  An extension has to ensure
that it works with multiple Flask application instances at once.  This is
a requirement because many people will use patterns like the
:ref:`app-factories` pattern to create their application as needed to aid
unittests and to support multiple configurations.  Because of that it is
crucial that your application supports that kind of behavior.

Most importantly the extension must be shipped with a :file:`setup.py` file and
registered on PyPI.  Also the development checkout link should work so
that people can easily install the development version into their
virtualenv without having to download the library by hand.

Flask extensions must be licensed under a BSD, MIT or more liberal license
in order to be listed in the Flask Extension Registry.  Keep in mind
that the Flask Extension Registry is a moderated place and libraries will
be reviewed upfront if they behave as required.

"Hello Flaskext!"
-----------------

So let's get started with creating such a Flask extension.  The extension
we want to create here will provide very basic support for SQLite3.

First we create the following folder structure::

    flask-sqlite3/
        flask_sqlite3.py
        LICENSE
        README

Here's the contents of the most important files:

setup.py
````````

The next file that is absolutely required is the :file:`setup.py` file which is
used to install your Flask extension.  The following contents are
something you can work with::

    """
    Flask-SQLite3
    -------------

    This is the description for that library
    """
    from setuptools import setup


    setup(
        name='Flask-SQLite3',
        version='1.0',
        url='http://example.com/flask-sqlite3/',
        license='BSD',
        author='Your Name',
        author_email='your-email@example.com',
        description='Very short description',
        long_description=__doc__,
        py_modules=['flask_sqlite3'],
        # if you would be using a package instead use packages instead
        # of py_modules:
        # packages=['flask_sqlite3'],
        zip_safe=False,
        include_package_data=True,
        platforms='any',
        install_requires=[
            'Flask'
        ],
        classifiers=[
            'Environment :: Web Environment',
            'Intended Audience :: Developers',
            'License :: OSI Approved :: BSD License',
            'Operating System :: OS Independent',
            'Programming Language :: Python',
            'Topic :: Internet :: WWW/HTTP :: Dynamic Content',
            'Topic :: Software Development :: Libraries :: Python Modules'
        ]
    )

That's a lot of code but you can really just copy/paste that from existing
extensions and adapt.

flask_sqlite3.py
````````````````

Now this is where your extension code goes.  But how exactly should such
an extension look like?  What are the best practices?  Continue reading
for some insight.

Initializing Extensions
-----------------------

Many extensions will need some kind of initialization step.  For example,
consider an application that's currently connecting to SQLite like the
documentation suggests (:ref:`sqlite3`).  So how does the extension
know the name of the application object?

Quite simple: you pass it to it.

There are two recommended ways for an extension to initialize:

initialization functions:

    If your extension is called `helloworld` you might have a function
    called ``init_helloworld(app[, extra_args])`` that initializes the
    extension for that application.  It could attach before / after
    handlers etc.

classes:

    Classes work mostly like initialization functions but can later be
    used to further change the behavior.  For an example look at how the
    `OAuth extension`_ works: there is an `OAuth` object that provides
    some helper functions like `OAuth.remote_app` to create a reference to
    a remote application that uses OAuth.

What to use depends on what you have in mind.  For the SQLite 3 extension
we will use the class-based approach because it will provide users with an
object that handles opening and closing database connections.

When designing your classes, it's important to make them easily reusable
at the module level. This means the object itself must not under any
circumstances store any application specific state and must be shareable
between different applications.

The Extension Code
------------------

Here's the contents of the `flask_sqlite3.py` for copy/paste::

    import sqlite3
    from flask import current_app, _app_ctx_stack


    class SQLite3(object):
        def __init__(self, app=None):
            self.app = app
            if app is not None:
                self.init_app(app)

        def init_app(self, app):
            app.config.setdefault('SQLITE3_DATABASE', ':memory:')
            app.teardown_appcontext(self.teardown)

        def connect(self):
            return sqlite3.connect(current_app.config['SQLITE3_DATABASE'])

        def teardown(self, exception):
            ctx = _app_ctx_stack.top
            if hasattr(ctx, 'sqlite3_db'):
                ctx.sqlite3_db.close()

        @property
        def connection(self):
            ctx = _app_ctx_stack.top
            if ctx is not None:
                if not hasattr(ctx, 'sqlite3_db'):
                    ctx.sqlite3_db = self.connect()
                return ctx.sqlite3_db


So here's what these lines of code do:

1.  The ``__init__`` method takes an optional app object and, if supplied, will
    call ``init_app``.
2.  The ``init_app`` method exists so that the ``SQLite3`` object can be
    instantiated without requiring an app object.  This method supports the
    factory pattern for creating applications.  The ``init_app`` will set the
    configuration for the database, defaulting to an in memory database if
    no configuration is supplied.  In addition, the ``init_app`` method
    attaches the ``teardown`` handler.
3.  Next, we define a ``connect`` method that opens a database connection.
4.  Finally, we add a ``connection`` property that on first access opens
    the database connection and stores it on the context.  This is also
    the recommended way to handling resources: fetch resources lazily the
    first time they are used.

    Note here that we're attaching our database connection to the top
    application context via ``_app_ctx_stack.top``. Extensions should use
    the top context for storing their own information with a sufficiently
    complex name.

So why did we decide on a class-based approach here?  Because using our
extension looks something like this::

    from flask import Flask
    from flask_sqlite3 import SQLite3

    app = Flask(__name__)
    app.config.from_pyfile('the-config.cfg')
    db = SQLite3(app)

You can then use the database from views like this::

    @app.route('/')
    def show_all():
        cur = db.connection.cursor()
        cur.execute(...)

Likewise if you are outside of a request you can use the database by
pushing an app context::

    with app.app_context():
        cur = db.connection.cursor()
        cur.execute(...)

At the end of the ``with`` block the teardown handles will be executed
automatically.

Additionally, the ``init_app`` method is used to support the factory pattern
for creating apps::

    db = SQLite3()
    # Then later on.
    app = create_app('the-config.cfg')
    db.init_app(app)

Keep in mind that supporting this factory pattern for creating apps is required
for approved flask extensions (described below).

.. admonition:: Note on ``init_app``

   As you noticed, ``init_app`` does not assign ``app`` to ``self``.  This
   is intentional!  Class based Flask extensions must only store the
   application on the object when the application was passed to the
   constructor.  This tells the extension: I am not interested in using
   multiple applications.

   When the extension needs to find the current application and it does
   not have a reference to it, it must either use the
   :data:`~flask.current_app` context local or change the API in a way
   that you can pass the application explicitly.


Using _app_ctx_stack
--------------------

In the example above, before every request, a ``sqlite3_db`` variable is
assigned to ``_app_ctx_stack.top``.  In a view function, this variable is
accessible using the ``connection`` property of ``SQLite3``.  During the
teardown of a request, the ``sqlite3_db`` connection is closed.  By using
this pattern, the *same* connection to the sqlite3 database is accessible
to anything that needs it for the duration of the request.


Learn from Others
-----------------

This documentation only touches the bare minimum for extension development.
If you want to learn more, it's a very good idea to check out existing extensions
on the `PyPI`_.  If you feel lost there is still the `mailinglist`_ and the
`Discord server`_ to get some ideas for nice looking APIs.  Especially if you do
something nobody before you did, it might be a very good idea to get some more
input.  This not only generates useful feedback on what people might want from
an extension, but also avoids having multiple developers working in isolation
on pretty much the same problem.

Remember: good API design is hard, so introduce your project on the
mailing list, and let other developers give you a helping hand with
designing the API.

The best Flask extensions are extensions that share common idioms for the
API.  And this can only work if collaboration happens early.

Approved Extensions
-------------------

Flask previously had the concept of approved extensions. These came with
some vetting of support and compatibility. While this list became too
difficult to maintain over time, the guidelines are still relevant to
all extensions maintained and developed today, as they help the Flask
ecosystem remain consistent and compatible.

0.  An approved Flask extension requires a maintainer. In the event an
    extension author would like to move beyond the project, the project
    should find a new maintainer and transfer access to the repository,
    documentation, PyPI, and any other services. If no maintainer
    is available, give access to the Pallets core team.
1.  The naming scheme is *Flask-ExtensionName* or *ExtensionName-Flask*.
    It must provide exactly one package or module named
    ``flask_extension_name``.
2.  The extension must be BSD or MIT licensed. It must be open source
    and publicly available.
3.  The extension's API must have the following characteristics:

    -   It must support multiple applications running in the same Python
        process. Use ``current_app`` instead of ``self.app``, store
        configuration and state per application instance.
    -   It must be possible to use the factory pattern for creating
        applications. Use the ``ext.init_app()`` pattern.

4.  From a clone of the repository, an extension with its dependencies
    must be installable with ``pip install -e .``.
5.  It must ship a testing suite that can be invoked with ``tox -e py``
    or ``pytest``. If not using ``tox``, the test dependencies should be
    specified in a ``requirements.txt`` file. The tests must be part of
    the sdist distribution.
6.  The documentation must use the ``flask`` theme from the
    `Official Pallets Themes`_. A link to the documentation or project
    website must be in the PyPI metadata or the readme.
7.  For maximum compatibility, the extension should support the same
    versions of Python that Flask supports. 3.6+ is recommended as of
    2020. Use ``python_requires=">= 3.6"`` in ``setup.py`` to indicate
    supported versions.

.. _PyPI: https://pypi.org/search/?c=Framework+%3A%3A+Flask
.. _OAuth extension: https://pythonhosted.org/Flask-OAuth/
.. _mailinglist: https://mail.python.org/mailman/listinfo/flask
.. _Discord server: https://discord.gg/t6rrQZH
.. _Official Pallets Themes: https://pypi.org/project/Pallets-Sphinx-Themes/