How to completely turn off caching? I am developing a site in Drupal 7 and caching slows my work significantly. For example, I had a theming function for a form. I was puzzled, why doesn't it work. Until I cleared the cache and it was fixed.

How to turn it off completely?
You can replace the default cache backend to one which does not actually do anything.
Simply add this to your settings.php file:

if (!class_exists('DrupalFakeCache')) {
  $conf['cache_backends'][] = 'includes/cache-install.inc';
}
// Default to throwing away cache data.
$conf['cache_default_class'] = 'DrupalFakeCache';
// Rely on the DB cache for form caching - otherwise forms fail.
$conf['cache_class_cache_form'] = 'DrupalDatabaseCache';


Just to be sure, set the cache expiration time to none on the Performance page.

I actually use this in a settings.local.php file which, if it exists, gets included at the bottom of my settings.php file so I can sync settings.php from the live or staging environments to my dev environment without having to worry about using the wrong database settings, accidentally pushing dev settings to live, etc.

settings.php:

if (file_exists(__DIR__ . '/settings.local.php')) {
  require_once __DIR__ . '/settings.local.php';
}