Understanding the url Tag Behavior with string_if_invalid

Based on the Django 5.1 documentation, you've correctly identified the different behaviors of the {% url %} template tag:

1. When used directly, it raises a NoReverseMatch exception if the URL doesn't exist
2. When used with the as var syntax, it silently assigns an empty string to the variable

This behavior is intentional and documented. The Django documentation specifically states:

This {% url ... as var %} syntax will not cause an error if the view is missing. In practice you'll use this to link to views that are optional.

Relationship with string_if_invalid

The string_if_invalid setting is designed to handle invalid variables that are being rendered in templates, not to control the behavior of template tags like {% url %}. The documentation explains:

Generally, if a variable doesn't exist, the template system inserts the value of the engine's string_if_invalid configuration option, which is set to '' (the empty string) by default.

This setting primarily affects how the template system handles variables that don't exist when you try to render them, not how template tags operate internally.

Practical Usage

The current behavior of the {% url %} tag with as var syntax is actually quite useful for conditional linking:

DJANGOCopied
{% url 'some-url-name' as the_url %}
{% if the_url %}
    <a href="{{ the_url }}">Link to optional stuff</a>
{% endif %}

This pattern allows you to conditionally include links only when the URL can be successfully reversed, which is a common requirement in templates.

Testing Considerations

For testing purposes, you're right that this behavior might make it harder to catch URL reversal issues during tests. The string_if_invalid setting won't help here since it's not involved in the {% url %} tag's internal behavior when using the as var syntax.

If you want to catch these issues in tests, you might need to:

1. Avoid using the as var syntax in templates that you want to strictly validate
2. Write specific tests that attempt to reverse your URLs directly
3. Use custom template tags or context processors that validate URLs before they reach the template

The pytest-django settings you mentioned (FAIL_INVALID_TEMPLATE_VARS and --fail-on-template-vars) are helpful for catching undefined variables, but they won't affect the intentional behavior of the {% url %} tag with as var syntax.