flask is a micro-web-framework in python. I have been using it for different projects for a couple of years now and I am quite happy with it.

I have been using it for some of the applications ran by the Fedora Infrastructure. Some of these applications could be re-used outside Fedora and this is of course something I would like to encourage.

One of the problem currently is that all those apps are branded for Fedora, so re-using them elsewhere can become complicated, this can be solved by theming. Theming means adjusting two components: templates and static files (images, css...).

Adjusting templates

jinja2 the template engine in flask already supports loading templates from two different directories. This allows to ask the application to load your own template first and if it does not find them, then it looks for it in the directory of the default theme.

Code wise it could look like this:

    # Use the templates
    # First we test the core templates directory
    # (contains stuff that users won't see)
    # Then we use the configured template directory
    import jinja2
    templ_loaders = []
    # First load the templates from the THEME_FOLDER defined in the configuration
        APP.root_path, APP.template_folder, APP.config['THEME_FOLDER'])))
    # Then load the other templates from the `default` theme folder
        APP.root_path, APP.template_folder, 'default')))
    APP.jinja_loader = jinja2.ChoiceLoader(templ_loaders)
Adjusting static files

This is a little more tricky as static files are not templates and there is no logic in flask to allow overriding one or another depending on where it is located.

To solve this challenge, I wrote a small flask extension: flask-multistatic that basically allows flask to have the same behavior for static files as it does for templates.

Getting it to work is easy, at the top of your flask application do the imports:

    import flask
    from flask_multistatic import MultiStaticFlask

And make your flask flask application multistatic

    APP = flask.Flask(__name__)
    APP = MultiStaticFlask(APP)

You can then specify multiple folders where static files are located, for example:

    APP.static_folder = [
        os.path.join(APP.root_path, 'static', APP.config['THEME_FOLDER']),
        os.path.join(APP.root_path, 'static', 'default')

Note: The order the the folder matters, the last one should be the folder with all the usual files (ie: the default theme), the other ones are the folders for your specific theme(s).

Patrick Uiterwijk pointed to me that this method, although working is not ideal for production as it means that all the static files are served by the application instead of being served by the web-server. He therefore contributed an example apache configuration allowing to obtain the same behavior (override static files) but this time directly in apache!

So using flask-multistatic I will finally be able to make my apps entirely theme-able, allowing other projects to re-use them under their own brand.