“pre_get_posts” firing on every query

Question

How can I change arguments for the main query only, and not affect other queries?

add_filter('pre_get_posts', 'custom_post_count');
function custom_post_count($query){
  $query->set('posts_per_page', 5);
  return $query;
};

Because this action is called inside the get_posts method of WP_Query, this code will alter the posts_per_page argument for all loops, not just the main, so passing this argument to WP_Query is useless...

Answer 1

Basically what you are looking for is the global $wp_the_query variable which is set to the value of the main query. It may not be a perfect fit for 100% of cases but will probably work fine in 99% of cases:

add_action( 'pre_get_posts', 'custom_post_count' );
function custom_post_count( $query ){
  global $wp_the_query;
  if ( $wp_the_query === $query ) {
    $query->set( 'posts_per_page', 5 );
  }
  return $query;
};

Answer 2

Firstly, 'pre_get_posts' is an action and not a filter. That's the main problem to start. Secondly, you need to set conditionals for the context.

add_action('wp', 'custom_post_count');
function custom_post_count($query){
    if($query->is_home || $query->is_front_page){
        $query->set('posts_per_page', 5);
    }
    return $query;
};

The previous example is if you want to use this once in your functions.php without touching your template files. As far as affecting every query, if you don't create a new query, every loop with inherit the pre_get_posts $query. That's why I use query_posts() to create a new query in the following example.

Custom Loops

This is how I do custom loops:

$args = array(
    'posts_per_page' => 5
);
query_posts($args);

if(have_posts()): while(have_posts()): the_post();


endwhile; else:

endif;

wp_reset_query();

Just place query_posts() above the loop and wp_reset_query() at the end of the loop.

Hope this helps you out. :)