Is it possible preprocess variables only for certain blocks? I created mytheme_preprocess_block__aggregator(&$vars), but it doesn't work.

  • The edit you did and the link you pasted there seems to only deal with theme hook suggestions, doesn't it?
    – leymannx
    Nov 17, 2016 at 20:56

3 Answers 3


Unfortunately, there is no way to do it like that (similar to hook_form_alter()).

The best way to do this would be to use $variables['block']->bid to apply modifications only to the blocks you want:

function mytheme_preprocess_block(&$variables) {
  if ($variables['block']->bid === 'target_block_id') {
    // do something for this block
  } else if ($variables['block']->bid === 'other_target_block_id') {
    // do something else for this other block
  • 4
    I looked at Drupal code, and Drupal doesn't looks for hook_preprocess_block_MODULE() or THEME_preprocess_block_MODULE(). It handle in a particular way the __ contained in theme function names, when theme() is invoked as theme('links__contextual__node', ...), for example.
    – apaderno
    Apr 7, 2011 at 15:37
  • Hmm, good to know!
    – Alex Weber
    Apr 7, 2011 at 17:09
  • 'block_id' is usually the machine name of the block correct?
    – chrisjlee
    Sep 26, 2011 at 14:46
  • 2
    Please make use of $variables['block']->bid and not $variables['block_id'] as 'block_id' is not unique to that block.
    – Duncanmoo
    Sep 5, 2013 at 9:42
  • 3
    I think its a little friendlier to conditionally switch on $variables['block']->delta if $variables['block']->module == 'MODULE'
    – Pete
    May 16, 2014 at 0:02

Just to confirm, in Drupal 8 you can write preprocess functions for specific blocks. For example:

Drupal 8

mytheme_preprocess_block__system_branding_block(&$vars) {
  // Make changes to the the system branding block

But you could also use hook_preprocess_block, and the plugin ID:

function mytheme_preprocess_block(&$vars) {
  if ($vars['plugin_id'] == 'system_branding_block') {
    // Make changes to the the system branding block

As mentioned by Alex, in Drupal 7 you'll have to stick with HOOK_preprocess_block, and an id check:

Drupal 7

mytheme_preprocess_block(&$vars) {
  if ($vars['block']->bid === 'target_block_id') {
    // make changes to this block
  • Tried it out right now on D8, neither MYTHEME_preprocess_block__system_branding_block(&$vars) works nor does MYTHEME_preprocess_block__page_title_block(&$variables).
    – leymannx
    Nov 15, 2016 at 12:24
  • 3
    mytheme_preprocess_block__{my_block_machine_name}(&$variables) does work on D8.3
    – Tim
    Mar 17, 2017 at 16:43

In Drupal 10 (presumably 8 & beyond), you can dig into the $vars array to isolate a particular block type/bundle:

function mytheme_preprocess_block(&$vars){
  $block_content = $vars['elements']['content']['#block_content'];
  if($block_content instanceof \Drupal\block_content\Entity\BlockContent) {
    $block_type = $block_content->bundle();
    // Logic here based on a (custom) block type/bundle.
    /* A variety of system-generated blocks (logo, menu, etc.) 
       will have other kinds of info in $vars['elements']['content'] 
       you may be able to key off of.
       Views-generated blocks will also provide details 
       about the View and display in $vars['elements']['content'].

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