How to Correctly Reference Template Variables within Jinja2's `url_for()` Function?

Referencing Template Variables within Jinja Expressions

The Problem

Consider the following Jinja2 template snippet:

<a href="{{ url_for('/magic/{{ filename }}') }}">Click to see magic happen</a>

This code attempts to generate a URL to a route defined as:

@app.route('/magic/<filename>')
def moremagic(filename):
    pass

However, the URL generated by the template snippet is incorrect because the {{ filename }} variable is not properly referenced within the url_for() function.

The Solution

To resolve this issue, the extra set of curly braces within the url_for() function must be removed. This is because everything within the {{ ... }} in Jinja2 is a Python-like expression, and therefore, it is not necessary to use another {{ ... }} to reference variables.

The corrected code is as follows:

<a href="{{ url_for('moremagic', filename=name) }}">Click to see magic happen</a>

Here, the name variable is passed as an argument to the url_for() function, and the endpoint name moremagic is used instead of the URL path.

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