How to Properly Reference Template Variables within Jinja `url_for` Expressions in Flask?

Referencing Template Variables within Jinja Expressions

In Flask web applications, it's common to dynamically generate URLs using template variables within Jinja expressions. However, referencing variables inside Jinja expressions can be challenging, as demonstrated in the following scenario:

Consider a route defined as:

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

To generate a link to the route from a template, you might want to use url_for() like this:

<h1>you uploaded {{ name }}</h1>
<a href="{{ url_for('/magic/<filename>') }}">Click to see magic happen</a>

However, if you need to dynamically insert the value of {{name}} into the URL, this approach results in a Jinja2 syntax error.

The solution to this issue lies in understanding the behavior of Jinja expressions. Inside {{ ... }}, everything is treated as Python-like expressions. Therefore, you don't need to use nested {{ ... }} to reference variables.

<h1>you uploaded {{ name }}</h1>
<a href="{{ url_for('moremagic', filename=name) }}">Click to see magic happen</a>

In this corrected code, the extra brackets around the Jinja expression are removed, and the name variable is passed as an argument to the url_for() function.

Note that url_for() requires the endpoint name, which is the name of the function (moremagic) in this case. The URL path itself is not required.

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