Get post Number with local loop and template

Question

Following on from my previous question: (Calling a custom excerpt function in a local loop ), I am not understanding why echo $wp_query->current_post; in template.php is returning -1 for every post when it seems to me it is in the loop and should each individual post number?

I need to get the post number of each post in the loop so I can target a specific post and manipulate the excerpt. I have tested everything else and it seems to be working. Could anyone explain this behaviour to me and how I could return the post number for each post? Code below, thanks.

home.php

<div>
<?php

$args = array(
    'posts_per_page' => 5,
    'post_type' => 'post',
    'cat'=> 2,
    'wpse_query_id' => 'home_post_loop' // Special parameter
);

$query = new WP_Query( $args );

if ( $query->have_posts() ) {
while ( $query->have_posts() ) {
     $query->the_post();
        get_template_part( 'template-parts/content', 'template' );
    }
        wp_reset_postdata();
}
?>
</div>

template.php

<div class="excerpts">

<div class="post-text">
<div class="entry-content">

    <span class="entry-text">   
        <?php
            the_excerpt();

            echo $wp_query->current_post; 
?>
    </span>
</div><!-- .entry-content -->

</div>
</div>

functions.php

<?php 
    add_action( 'pre_get_posts', 'wpse_excerpt_length_handler' );
function wpse_excerpt_length_handler( $query ) {
    if ( isset( $query->query['wpse_query_id'] ) && 'home_post_loop' === $query->query['wpse_query_id'] ) {     
        add_filter( 'excerpt_length', 'wpse_custom_excerpt_loop', 999 );


function wpse_custom_excerpt_loop( $length ) {
    global $wp_query;

    if ( $wp_query->current_post == 0 && ! $wp_query->is_paged ) {
        $length = 20;
    } else {
        $length = 10;
    }

    return $length;
}


  ?>