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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 `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 setup.py files.

Flask sets up a redirect package called `flask.ext` where users should import the extensions from. If you for instance have a package called `flask_something` users would import it as `flask.ext.something`. This is done to transition from the old namespace packages. See Extension Import Transition for more details.

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 Application 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 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 to be able to be enlisted 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 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 (Using SQLite 3 with Flask). 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.

What’s important about classes is that they encourage to be shared around on module level. In that case, the object itself must not under any circumstances store any application specific state and must be shareable between different application.

## The Extension Code

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

    import sqlite3
    from flask import current_app

    # Find the stack on which we want to store the database connection.
    # Starting with Flask 0.9, the _app_ctx_stack is the correct one,
    # before that we need to use the _request_ctx_stack.
    try:
        from flask import _app_ctx_stack as stack
    except ImportError:
        from flask import _request_ctx_stack as 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:')
            # Use the newstyle teardown_appcontext if it's available,
            # otherwise fall back to the request context
            if hasattr(app, 'teardown_appcontext'):
                app.teardown_appcontext(self.teardown)
            else:
                app.teardown_request(self.teardown)

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

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

        @property
        def connection(self):
            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. It will try to use the newstyle app context handler and if it does not exist, falls back to the request context one.

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. Note that we’re falling back to the `_request_ctx_stack.top` if the application is using an older version of Flask that does not support it.

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 but you are using Flask 0.9 or later with the app context support, you can use the database in the same way:

    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).

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 `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.

If the `_app_ctx_stack` does not exist because the user uses an old version of Flask, it is recommended to fall back to `_request_ctx_stack` which is bound to a request.

## Teardown Behavior

This is only relevant if you want to support Flask 0.6 and older

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:

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 including full source hosting transition and PyPI access. If no maintainer is available, give access to the Flask core team.
1. An approved Flask extension must provide exactly one package or module named `flask_extensionname`. They might also reside inside a `flaskext` namespace packages though this is discouraged now.
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. If tests are invoked with `python setup.py test`, test dependencies 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 2.6 as well as Python 2.7

## Extension Import Transition

For a while we recommended using namespace packages for Flask extensions. This turned out to be problematic in practice because many different competing namespace package systems exist and pip would automatically switch between different systems and this caused a lot of problems for users.

Instead we now recommend naming packages `flask_foo` instead of the now deprecated `flaskext.foo`. Flask 0.8 introduces a redirect import system that lets uses import from `flask.ext.foo` and it will try `flask_foo` first and if that fails `flaskext.foo`.

Flask extensions should urge users to import from `flask.ext.foo` instead of `flask_foo` or `flaskext_foo` so that extensions can transition to the new package name without affecting users.