Automatically password protect all new custom post types in WordPress

password-protect.php

<?php 
add_filter( 'wp_insert_post_data', function( $data, $postarr ){
	if ( 'book' == $data['post_type'] && 'auto-draft' == $data['post_status'] ) {
		$data['post_password'] = wp_generate_password();
	}
	return $data;
}, '99', 2 );

This will set an automatically generated password for each new post of the type "book" in WordPress. Very handy if you want all your content to be password protected and make sure you don't accidentally publish it without one.