During build: string concatenation in url_for() fails when data comes from render_template()

[LokiWijnen]

When running Flask normally, everything described below works absolutely fine.

When running a build in Frozen-Flask however, I run into the issue where my dynamically built route string won't allow me to concatenate a variable (containing a string) with a string for use in a url_for() function within Jinja2. But only when the variable data is originating from Python such as render_template().

routes.py

@app.route('/some-route/')
def some_route():
    return render_template('my_template.html', foo='bar')

base.html

{% import "includes.html" as includes %}
{{ includes.my_macro(foo) }}

includes.html

# The following does NOT work
{% macro my_macro(foo) %}
    <a href="{{ url_for('index_' ~ foo) }}">Link text</a>
{% endmacro %}

Although the above runs fine in Flask, when I do a build with Frozen-Flask I get the following error:

werkzeug.routing.BuildError: Could not build url for endpoint 'index_'. Did you mean...

As you can see, the value of foo (or the string value bar) is missing. It should have been index_bar.

So I tried this instead:

# This also does NOT work
{% macro my_macro(foo) %}
    {% set route = 'index_' ~ foo %}
    <a href="{{ url_for(route) }}">Link text</a>
{% endmacro %}

The above produces the exact same error.

So I tried this to try to better understand the problem:

# This works correctly
{% macro my_macro(foo) %}
    {% set route = 'index_' ~ 'bar' %}
    <a href="{{ url_for(route) }}">Link text</a>
{% endmacro %}

So I further tried this:

# This also works correctly
{% macro my_macro(foo) %}
    {% set foo2 = 'bar' %}
    {% set route = 'index_' ~ foo2 %}
    <a href="{{ url_for(route) }}">Link text</a>
{% endmacro %}

So basically, I can create a dynamic route string for use by url_for() but only when the variable data doesn't come from the Flask routes file (such as my routes.py), only if the variable is created within Jinja2.

[TomHall2020]

Your routes.py file in the example only defines some_route. I'm trying to understand your setup here, do you actually have multiple routes with endpoints index_bar, index_other etc? If so you may want to create a true dynamic route as follows:

@app.route('/somewhere/<foo>')
def somewhere(foo):
    return render_template('my_template.html', foo=foo)

then in your templates you can access these via url_for as url_for(somewhere, foo='bar') and skip out the macro and string concatenation in the template.

[LokiWijnen]

I am aware that I can do this:

url_for(somewhere, foo='bar')

But this isn't the problem that I am trying to solve.

---

The issue (ignoring jinja macros for this explanation) is that url_for will not accept a concatenate a string with a variable originating in Flask when running a Build, but it works fine when running Flask normally.

So, for example if the value coming from Flask is language = 'en' like this:

return render_template('my_template.html', language='en')

Then in Jinja, I can't do this:

{{ url_for('index_' ~ language) }}

To get it to work, I have to do something like this where I create a Jinja variable:

{% set jinja_language = 'de' if language == 'de' else 'en' %}

{{ url_for('index_' ~ jinja_language) }}