Flask Extension System

Posted on with tags: flask

Flask is a micro web framework, and many functionalities are provided by various extensions. The Flask documentation only has two pages on extensions, one is on how to use extensions and the other is about how to develop new extensions.


Let’s look at the source code of some popular extensions in this article. The first one is Flask-Bootstrap. When you read Miguel Grinberg’s Flask Web Development book, you will find that Flask-Bootstrap not only helps Flask integrating with Bootstrap, it also helps integrating Bootstrap with WTForms and provides a quick_form function. You can render a form like this.

{% import "bootstrap/wtf.html" as wtf %}
{{ wtf.quick_form(form) }}

The drawback of this extension is that it has not been updated for sometime. The Bootstrap 4 has been released a while, but the extension still includes the version 3. There is a fork Flask-BS4 with Bootstrap 4, but it is not as popular as Flask-Bootstrap.

After you pip install Flask-Bootstrap in a virtual environment, the files are installed in this directory.


The directory has three Python files __init__.py, forms.py, and nav.py. It also has two directories static and templates. The directory structure looks like this.

├── forms.py
├── __init__.py
├── nav.py
├── static
│   ├── css
│   │   ├── bootstrap.css
│   │   ├── ....
│   ├── fonts
│   │   ├── glyphicons-halflings-regular.eot
│   │   ├── ....
│   ├── jquery.js
│   ├── jquery.min.js
│   ├── jquery.min.map
│   └── js
│       ├── bootstrap.js
│       ├── bootstrap.min.js
│       └── npm.js
└── templates
    └── bootstrap
        ├── base.html
        ├── fixes.html
        ├── google.html
        ├── pagination.html
        ├── utils.html
        └── wtf.html

When you use the Flask-Bootstrap in an app, you initialize it with those two lines.

from flask_bootstrap import Bootstrap
bootstrap = Bootstrap(app)

The Bootstrap class is defined on Line 123 (version of __init__.py file. The init_app method does the actual initialization work.

def init_app(self, app):
    app.config.setdefault('BOOTSTRAP_USE_MINIFIED', True)

    blueprint = Blueprint(
        static_url_path=app.static_url_path + '/bootstrap',

    # add the form rendering template filter


    app.jinja_env.globals['bootstrap_is_hidden_field'] =\
    app.jinja_env.globals['bootstrap_find_resource'] =\

    if not hasattr(app, 'extensions'):
        app.extensions = {}

The interesting part of the init_app method is that it registers itself as a blueprint (sub application) of the app. After you initialize the extension, the app will search template and static files in this extension.


Flask-Moment is introduced in Chapter 3 (pages 38-41) of the Flask Web Development book. The extension is actually created by Miguel Grinberg. The extension makes Moment.js Javascript library easy to use in Flask.

The Flask-Moment is simple, and it only has one file flask_moment.py which is 198 lines long. The Moment class code is shown below.

class Moment(object):
    def __init__(self, app=None):
        if app is not None:

    def init_app(self, app):
        if not hasattr(app, 'extensions'):
            app.extensions = {}
        app.extensions['moment'] = _moment

    def context_processor():
        return {
            'moment': current_app.extensions['moment']

    def create(self, timestamp=None):
        return current_app.extensions['moment'](timestamp)

The context_processor static method returns a dictionary. The key (moment) is the variable or method name used in template, and the value (current_app.extensions[moment] refers to _moment) is the actual Python variable or method. The implementation here is a little difficult to understand at a glance. The _moment is actually a class.

class _moment(object):
    def include_moment(version=default_moment_version, 
                       no_js=None, sri=None):
        js = ''

    def __init__(self, timestamp=None, local=False):
        if timestamp is None:
            timestamp = datetime.utcnow()
        self.timestamp = timestamp
        self.local = local

    def fromNow(self, no_suffix=False, refresh=False):
        return self._render("fromNow", 

Let’s look at an example in a template file.

{{ moment(current_time).fromNow(refresh=True) }}

When you have this template variable in your template file, the moment(current_time) part is actually calling the __init__ method of _moment class. The return value is an instance of _moment class. The fromNow method is called on this instance. It then calls the _render method, which returns a <span> html element which has a class attribute flask-moment.

def _render(self, func, format=None, timestamp2=None, 
        t = self._timestamp_as_iso_8601(self.timestamp)
        return Markup((
            '<span class="flask-moment" data-timestamp="{}" ' +
            '{} data-refresh="{}" ' +
            'style="display: none">{}</span>').format(
            t, data_values, int(refresh) * 60000, t))

The include_moment static method of _moment class includes a CDN link for the Moment Javascript library and three functions to render html elements with flask-moment class attribute.

This include_moment method is quite interesting because the Python file includes Javascript code quoted as a multi-line string. When Jinja2 renders the template file, the string is rendered as Javascript code on the html file. The html file is then transmitted to the client browser, and finally the browser runs the Javascript code originated in a Python file.