How to use a Jinja2 extension direct from a Flask render_template

Question

I'm working on a Flask app that is working just fine, until I try to add the following line to a template to be rendered:

{% do totals.update({tier: 0}) %}

The current code for rendering the template uses Flask's render_template():

from flask import Flask, Response, request, session
from flask import render_template
app = Flask(__name__)

..
return render_template(<template.htlm>,...)

This fails with with following error:

TemplateSyntaxError: Encountered unknown tag 'do'. Jinja was looking for the following tags: 'endfor' or 'else'. The innermost block that needs to be closed is 'for'.

The obvious fix is to add the jinja2.ext.do extension to jinja. I have been able to do so successfully using Jinja2 directly, as per:

from jinja2 import Environment, PackageLoader
ENV = Environment(loader=PackageLoader('ticket_app', 'templates'), extensions=['jinja2.ext.do'])
...
TEMP = ENV.get_template('div_' + div_id + '.html')
return TEMP.render(sales_new=sales_new, event_config=event_config)

However, I would prefer to not use Jinja2 directly... The app was only using Flask and render_template() before, and as render_template() uses Jinja2 under the hood (as far as I understand) it seems that it should be possible to make render_template() understand the jinja2.ext.do extension (or any other extension for that matter).

So far, I've tried the following:

app = Flask(__name__)
app.config['TEMPLATES_AUTO_RELOAD'] = True
app.config['EXPLAIN_TEMPLATE_LOADING'] = True
env = app.jinja_env
env.add_extension('jinja2.ext.do')

While the above does not throw an error, it is also not causing it to make render_template() understand the jinja2.ext.do extension.

Any suggestions? Should this be possible? If so, how?

Answer 1

Update for Flask v.2.0 :

Since v.2.0, the below solution raise a KeyError: 'extensions' exception. Try this instead.

---

For Flask v.1.1, you can directly access to the Jinja extensions loaded by Flask with the Flask.jinja_options dictionary. In your case, adding just this line should do the trick:

app = Flask(__name__)
app.jinja_options['extensions'].append('jinja2.ext.do')

Make sure you update your Flask (using pip: pip install -U Flask).