"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 ?

Alan Hamlett · Accepted Answer · 2022-12-10T19:01:23.127

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 }}

edited Dec 10 '22 at 19:01

answered Feb 09 '18 at 04:20

Alan Hamlett

3,160
1
23
23

The `render()` method on `Markup(...)` did not work for me, I think it does not have that method. – Andreas Maier Dec 10 '22 at 15:12
@AndreasMaier thanks, updated it to work with the latest Flask, Jinja2, and markupsafe. – Alan Hamlett Dec 10 '22 at 19:02

score 0 · Answer 2 · answered Feb 28 '18 at 20:03

0

I was looking for a similar eval use case and bumped into a different stack overflow post.

This worked for me

routes.py

def index():
    html = "<b>actual eval string</b>"
    return render_template('index.html', html_str = html)

index.html

<html>
    <head>
        <title>eval jinja</title>
    </head>
    <body>
        {{ html_str | safe }}
    </body>
</html>

Reference : Passing HTML to template using Flask/Jinja2

answered Feb 28 '18 at 20:03

Naveen Vijay

15,928
7
71
92

1

This example does not evaluate a Jinja2 expression, which is what the OP had asked for. – Andreas Maier Dec 10 '22 at 15:23

score 0 · Answer 3 · answered Nov 13 '22 at 10:39

0

If you are using jinja version 3.0, contextfilter has deprecated and removed in Jinja 3.1. Use pass_context() instead. Refer here for more details pertaining this.

from jinja2 import pass_context, Template
from markupsafe import Markup


@pass_context
def foo(context, value):
    return Markup(Template(value).render(context)).render()

then in your template

 {{ myvar|foo }}

answered Nov 13 '22 at 10:39

Hilario Nengare

154
6

The `render()` method on `Markup(...)` did not work for me, I think it does not have that method. – Andreas Maier Dec 10 '22 at 15:13

score 0 · Answer 4 · answered Dec 10 '22 at 15:19

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
```

"eval" statement in Jinja2 template

4 Answers4