"eval" statement in Jinja2 template

Question

I'm trying to convert some old Smarty templates to Jinja2.

Smarty uses an eval statement in the templates to render a templated string from the current context.

Is there an eval equivalent in Jinja2 ? Or what is a good workaround for this case ?

Answer 1

Use the @jinja2.contextfilter decorator to make a Custom Filter for rendering variables:

from flask import render_template_string
from jinja2 import contextfilter
from markupsafe import Markup


@contextfilter
def dangerous_render(context, value):
    Markup(render_template_string(value, **context)).format()

Then in your template.html file:

{{ myvar|dangerous_render }}

Answer 2

Based on the answers from Alan Hamlett and Hilario Nengare, I implemented the following "eval" filter for Jinja2 (>=3.0). I found that the case of undefined input should also be handled, plus I added the ability to specify additional variables as input for the expression:

import jinja2

@jinja2.pass_context
def filter_eval(context, input, **vars):
    if input == jinja2.Undefined:
        return input
    return jinja2.Template(input).render(context, **vars)

Usage, when adding filter_eval() as filter "eval":

• Template source:

  {% set rack_str = 'Rack {{ rack }}' %}
  {{ rack_str | eval(rack='A') }}
  

• Rendered template:

  Rack A