Raise error for undefined attributes in Jinja templates in Flask

Question

By default Flask renders empty values for undefined attributes in Jinja templates. I want to raise an error instead. How can I change this behavior in Flask?

Hello, {{ name }}!

render_template('index.html')

Hello, !

Answer 1

Change the Flask app's Jinja env's undefined class to be StrictUndefined.

from flask import Flask
from jinja2 import StrictUndefined

app = Flask(__name__)
app.jinja_env.undefined = StrictUndefined

If a template tries to use a variable that is undefined (except to test if it's undefined) it will raise an error.

Hello, {{ name }}!

render_template('index.html')

jinja2.exceptions.UndefinedError: 'name' is undefined