Stop logging warnings from the |attr filter

[mmikkel]

TL;DR: It'd be great if the |attr filter could stop logging a warning if the input string doesn't contain any HTML tags.

This is a thing that has been bugging me for a while... 😇

If the attr Twig filter is unable to find an HTML tag in the input string, Craft will log a WARNING that tends to pollute logs if one isn't careful:

2025-07-16 13:19:47 [web.WARNING] [craft\web\twig\Extension::attrFilter] Could not find an HTML tag in string: Hello there. I am the rendered output from foo.twig.  {"memory":2717160}

I tend to write fairly defensive Twig, and typically will have a lot of "component" templates that won't render any HTML if some conditional isn't met. E.g. something like

{# _components/shortcut-menu.twig #}
{% set shortcutMenuItems = shortcutMenuItems ?? null %}

{% if shortcutMenuItems %}
    <nav>...</nav>
{% endif %}

I also like to use |attr a lot, often when including reusable Twig components, for example something like

{# _includes/header.twig #}
<header>
    {{ include('_components/shortcut-menu.twig', { shortcutMenuItems })|attr({ class: 'absolute left-0 top-0') }}
    
    ...
</header>

In the example above, I don't really care if the shortcut menu component rendered or not (maybe the client deleted all the shortcut menu items 🤷).

But, in order to avoid having warnings from |attr polluting the logs, I have to make sure that anything I'm using |attr on, actually contains HTML before I use the filter 😩

In the example above, that could mean wrapping the include() statement in a conditional, checking if shortcutMenuItems is empty or not (which is the same thing happening inside the template, so otherwise redundant in this case):

{% if shortcutMenuItems %}
    {{ include('_components/shortcut-menu.twig', { shortcutMenuItems })|attr({ class: 'absolute left-0 top-0') }}
{% endif %}

Which... isn't too bad, but feels a bit clumsy.

But, creating a conditional like that isn't always feasible. In some cases I might instead have to check if the output is empty, by using the |spaceless filter (now deprecated! 😰) – which is a lot more awkward, e.g.

{% set adWidget = include('_includes/ad-widget.twig') %}

{% if adWidget|spaceless %}
    {{ adWidget|attr({ class: 'fixed center' }) }}
{% endif %}

...it's a bit annoying. More to the point, it feels inconsistent with how Twig works generally, where filters are either very lenient about "invalid" input, or throws an actual exception.

For example, something like {{ 'Hamburgers are good'|round }} will not log a warning that the input is not a number – it will just return 0 🤷

On the flip side, doing something like {{ ['Hamburger!'] }} actually throws an exception (Array to string conversion), as Twig isn't able to reconcile the input to anything that can be rendered (I'm not suggesting the |attr filter starts doing that!).

To me, it feels like the |attr filter should behave like the |round example and not log a warning (especially not outside of devMode), in cases where it's used on something that doesn't actually contain HTML 🤷 This would allow the filter to be used a little less defensively and more in line with how other Twig filters work, without worrying about logs getting filled up by these non-errors.