When is wp_trim_excerpt() called?

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1. If you’ve already looked in XRef, you’ve seen this comment at the top of the doc block:

   /**
     * Generates an excerpt from the content, if needed.
   

   So it’s not used in core, but is available for you to use if you need it.

   Update

   Let me explain a little of what’s going on:

   excerpt_length and excerpt_more are inside wp_trim_excerpt, and changing them does in fact change the_excerpt and get_the_excerpt. So wp_trim_excerpt has to be doing something somewhere.

   Yes and no. excerpt_length and excerpt_more are being generated by filters. Here’s the inside of the wp_trim_excerpt() function:

   $excerpt_length = apply_filters('excerpt_length', 55);
   $excerpt_more = apply_filters('excerpt_more', ' ' . '[...]');
   

   By default, $excerpt_length will be set to 55 and $excerpt_more will be set to [...]. However, you can change this in themes and plugins. Let’s say I wanted to sent the length of trimmed excerpts to be 70 characters instead. I’d use the following code:

   function set_new_trim_length( $excerpt_length ) {
       return 70;
   }
   add_filter( 'excerpt_length', 'set_new_trim_length' );
   

   When I call wp_trim_excerpt() later, WordPress will pass that default value of 55 in to my filter function. My filter function will then pass 70 back and wp_trim_excerpt() will use that instead.

   wp_trim_excerpt() doesn’t set or change excerpt_length or excerpt_more when you call it. The function merely uses whatever filters you already have defined for those variables.

   Further Update

   But when you call the_excerpt or get_the_excerpt, they are modified by those filters, so what I want to know is ‘when’ they are modified by those filters.

   This all depends on how you’re building content. There’s a filter applied to get_the_excerpt() that will pass the generated content through the wp_trim_excerpt() function.

   So if you hand-write an excerpt, the hand-written content will be passed through unchanged.

   If you don’t write an excerpt ($post->post_excerpt = ''), the content of the post is passed through wp_trim_excerpt() insted, shortened, and returned.

   For reference, this filter is added on line 147 of /wp-includes/default-filters.php:

   add_filter( 'get_the_excerpt', 'wp_trim_excerpt'  );
   

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