How to dynamically load template parts in block themes

[bph]

Discussed in #459

^{Originally posted by justintadlock March 25, 2026}
Here's a quick snippet I used to dynamically load a category-based sidebar template for single posts:

add_filter('render_block_data', 'themeslug_render_template_part_data');

function themeslug_render_template_part_data(array $parsedBlock): array
{
	if (
		($parsedBlock['blockName'] ?? '') !== 'core/template-part'
		|| ($parsedBlock['attrs']['slug'] ?? '') !== 'sidebar-post'
		|| ! is_singular('post')
	) {
		return $parsedBlock;
	}

	$post = get_queried_object();

	if (! $post instanceof WP_Post) {
		return $parsedBlock;
	}

	$partsDir = get_block_theme_folders()['wp_template_part'];

	foreach (get_the_category($post->ID) as $category) {
		$slug = "sidebar-post-{$category->slug}";

		if (locate_template("{$partsDir}/{$slug}.html")) {
			$parsedBlock['attrs']['slug'] = $slug;
			return $parsedBlock;
		}
	}

	return $parsedBlock;
}

Essentially, it looks for parts/sidebar-post-{category}.html and falls back to parts/sidebar-post.html, allowing theme authors to create sidebars based on the post category.

There's loads of things that you can do with this, though. It's not limited to sidebars and could be realistically used to replace any template part based on the given data.

This is just an easy snippet, but I could see it being wrapped up into a longer tutorial.

[justintadlock]

Got a rough draft of a full tutorial (rather than snippet) here: https://docs.google.com/document/d/1g9TdgCVOU3XgLb1xJmQgFq08BvPHN47fB7-SStrEbjQ/edit?usp=sharing

Not ready for review yet, but I have more thoughts that aren't great for the snippet format.