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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.

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's 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 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 behaviour. 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 a manager object that handles opening and closing database connections.

The Extension Code

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

from __future__ import absolute_import
import sqlite3

from flask import _request_ctx_stack

class SQLite3(object):

    def __init__(self, app):
        self.app = app
        self.app.config.setdefault('SQLITE3_DATABASE', ':memory:')
        self.app.teardown_request(self.teardown_request)
        self.app.before_request(self.before_request)

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

    def before_request(self):
        ctx = _request_ctx_stack.top
        ctx.sqlite3_db = self.connect()

    def teardown_request(self, exception):
        ctx = _request_ctx_stack.top
        ctx.sqlite3_db.close()

    def get_db(self):
        ctx = _request_ctx_stack.top
        if ctx is not None:
            return ctx.sqlite3_db

So here's what these lines of code do:

  1. The __future__ import is necessary to activate absolute imports. 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 requires a supplied app object, sets a configuration for the database if it's not there (:meth:`dict.setdefault`), and attaches before_request and teardown_request handlers.
  3. Next, we define a connect function that opens a database connection.
  4. Then we set up the request handlers we bound to the app above. Note here that we're attaching our database connection to the top request context via _request_ctx_stack.top. Extensions should use the top context and not the g object to store things like database connections.
  5. Finally, we add a get_db function that simplifies access to the context's database.

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

from flask import Flask
from flaskext.sqlite3 import SQLite3

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

You can then use the database from views like this:

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

Opening a database connection from outside a view function is simple.

>>> from yourapplication import db
>>> cur = db.cursor()
>>> cur.execute(...)

Adding an init_app Function

In practice, you'll almost always want to permit users to initialize your extension and provide an app object after the fact. This can help avoid circular import problems when a user is breaking their app into multiple files. Our extension could add an init_app function as follows:

class SQLite3(object):

    def __init__(self, app=None):
        if app is not None:
            self.app = app
            self.init_app(self.app)
        else:
            self.app = None

    def init_app(self, app):
        self.app = app
        self.app.config.setdefault('SQLITE3_DATABASE', ':memory:')
        self.app.teardown_request(self.teardown_request)
        self.app.before_request(self.before_request)

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

    def before_request(self):
        ctx = _request_ctx_stack.top
        ctx.sqlite3_db = self.connect()

    def teardown_request(self, exception):
        ctx = _request_ctx_stack.top
        ctx.sqlite3_db.close()

    def get_db(self):
        ctx = _request_ctx_stack.top
        if ctx is not None:
            return ctx.sqlite3_db

The user could then initialize the extension in one file:

manager = SQLite3()

and bind their app to the extension in another file:

manager.init_app(app)

End-Of-Request Behavior

Due to the change in Flask 0.7 regarding functions that are run at the end of the request your extension will have to be extra careful there if it wants to continue to support older versions of Flask. The following pattern is a good way to support both:

def close_connection(response):
    ctx = _request_ctx_stack.top
    ctx.sqlite3_db.close()
    return response

if hasattr(app, 'teardown_request'):
    app.teardown_request(close_connection)
else:
    app.after_request(close_connection)

Strictly speaking the above code is wrong, because teardown functions are passed the exception and typically don't return anything. However because the return value is discarded this will just work assuming that the code in between does not touch the passed parameter.

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. This not only to get an idea about what people might want to have from an extension, but also to avoid having multiple developers working on pretty much the same side by side.

Remember: good API design is hard, so introduce your project on the mailinglist, 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 also has the concept of approved extensions. Approved extensions are tested as part of Flask itself to ensure extensions do not break on new releases. These approved extensions are listed on the Flask Extension Registry and marked appropriately. If you want your own extension to be approved you have to follow these guidelines:

  1. An approved Flask extension must provide exactly one package or module inside the flaskext namespace package.
  2. It must ship a testing suite that can either be invoked with make test or python setup.py test. For test suites invoked with make test the extension has to ensure that all dependencies for the test are installed automatically, in case of python setup.py test dependencies for tests alone can be specified in the setup.py file. The test suite also has to be part of the distribution.
  3. APIs of approved extensions will be checked for the following characteristics:
    • an approved extension has to support multiple applications running in the same Python process.
    • it must be possible to use the factory pattern for creating applications.
  4. The license must be BSD/MIT/WTFPL licensed.
  5. The naming scheme for official extensions is Flask-ExtensionName or ExtensionName-Flask.
  6. Approved extensions must define all their dependencies in the setup.py file unless a dependency cannot be met because it is not available on PyPI.
  7. The extension must have documentation that uses one of the two Flask themes for Sphinx documentation.
  8. The setup.py description (and thus the PyPI description) has to link to the documentation, website (if there is one) and there must be a link to automatically install the development version (PackageName==dev).
  9. The zip_safe flag in the setup script must be set to False, even if the extension would be safe for zipping.
  10. An extension currently has to support Python 2.5, 2.6 as well as Python 2.7
Something went wrong with that request. Please try again.