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

Flask, being a microframework, often requires some repetitive steps to get
a third party library working.  Because very often these steps could be
abstracted to support multiple projects the `Flask Extension Registry`_
was created.

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.

.. _Flask Extension Registry: http://flask.pocoo.org/extensions/

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

Extensions are all located in a package called ``flaskext.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 ``flaskext.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 `setup.py` files.

The magic that makes it possible to have your library in a package called
``flaskext.something`` is called a "namespace package".  Check out the
guide below how to create something like that.

But how 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 behaviour.

Most importantly the extension must be shipped with a `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 as BSD or MIT or a more liberal license
to be enlisted on 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.

There is a script on github called `Flask Extension Wizard`_ which helps
you create the initial folder structure.  But for this very basic example
we want to create all by hand to get a better feeling for it.

First we create the following folder structure::

    flask-sqlite3/
        flaskext/
            __init__.py
            sqlite3.py
        setup.py
        LICENSE

Here the contents of the most important files:

flaskext/__init__.py
````````````````````

The only purpose of this file is to mark the package as namespace package.
This is required so that multiple modules from different PyPI packages can
reside in the same Python package::

    __import__('pkg_resources').declare_namespace(__name__)

If you want to know exactly what is happening there, checkout the
distribute or setuptools docs which explain how this works.

Just make sure to not put anything else in there!

setup.py
````````

The next file that is absolutely required is the `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__,
        packages=['flaskext'],
        namespace_packages=['flaskext'],
        zip_safe=False,
        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.  This is also what the wizard creates for you if you
use it.

flaskext/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 your application is currently connecting to SQLite like the
documentation suggests (:ref:`sqlite3`) you will need to provide a few
functions and before / after request handlers.  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 initalizes 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 behaviour.  For an example look at how the
    `OAuth extension`_ works: ther 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 need to use the class based approach because we have to use a
controller object that can be used to connect to the database.

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

Here the contents of the `flaskext/sqlite3.py` for copy/paste::

    from __future__ import absolute_import
    import sqlite3
    from flask import g

    class SQLite3(object):
    
        def __init__(self, app):
            self.app = app
            self.app.config.setdefault('SQLITE3_DATABASE', ':memory:')

            self.app.before_request(self.before_request)
            self.app.after_request(self.after_request)

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

        def before_request(self):
            g.sqlite3_db = self.connect()

        def after_request(self, response):
            g.sqlite3_db.close()
            return response

So here what the lines of code do:

1.  the ``__future__`` import is necessary to activate absolute imports.
    This is needed because otherwise we could not call our module
    `sqlite3.py` and import the top-level `sqlite3` module which actually
    implements the connection to SQLite.
2.  We create a class for our extension that sets a default configuration
    for the SQLite 3 database if it's not there (:meth:`dict.setdefault`)
    and connects two functions as before and after request handlers.
3.  Then it implements a `connect` function that returns a new database
    connection and the two handlers.

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

    from flask import Flask, g
    from flaskext.sqlite3 import SQLite3

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

Either way you can use the database from the views like this::

    @app.route('/')
    def show_all():
        cur = g.sqlite3_db.cursor()
        cur.execute(...)

But how would you open a database connection from outside a view function?
This is where the `db` object now comes into play:

>>> from yourapplication import db
>>> con = db.connect()
>>> cur = con.cursor()

If you don't need that, you can go with initialization functions.

Initialization Functions
------------------------

Here how the module would look like with initialization functions::

    from __future__ import absolute_import
    import sqlite3
    from flask import g

    def init_sqlite3(app):
        app = app
        app.config.setdefault('SQLITE3_DATABASE', ':memory:')

        @app.before_request
        def before_request():
            g.sqlite3_db = sqlite3.connect(self.app.config['SQLITE3_DATABASE'])

        @app.after_request
        def after_request(response):
            g.sqlite3_db.close()
            return response

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 `Flask Extension Registry`_.  If you feel
lost there is still the `mailinglist`_ and the `IRC channel`_ 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.

Remember: good API design is hard :(


.. _Flask Extension Wizard:
   http://github.com/mitsuhiko/flask-extension-wizard
.. _OAuth extension: http://packages.python.org/Flask-OAuth/
.. _mailinglist: http://flask.pocoo.org/mailinglist/
.. _IRC channel: http://flask.pocoo.org/community/irc/