Importing Drupal 8 config programmatically

import.php

<?php

// Import arbitrary config from a variable.
// Assumes $data has the data you want to import for this config.
$config = \Drupal::service('config.factory')->getEditable('filter.format.basic_html');
$config->setData($data)->save();

// Or, re-import the default config for a module or profile, etc.
\Drupal::service('config.installer')->installDefaultConfig('module', 'my_custom_module');

// Or, import YAML config from an arbitrary file.
$config_path = drupal_get_path('module', 'my_custom_module') . '/config/install';
$source = new FileStorage($config_path);
$config_storage = \Drupal::service('config.storage');
$config_storage->write('filter.format.basic_html', $source->read('filter.format.basic_html'));

Very helpful, thank you!

You may also like this to install optional config e.g. in an update hook:

  $config_path = drupal_get_path('module', 'MY_MODULE') . '/config/optional';
  $config_source      = new \Drupal\Core\Config\FileStorage($config_path);
  \Drupal::service('config.installer')->installOptionalConfig($config_source);

If you want to import all the site configs (drush required):

  /* @var \Drush\Drupal\Commands\config\ConfigImportCommands $drush_import */
  $container = \Drupal::getContainer();
  $drush_import = $container->get('config.import.commands');
  $storage_comparer = new StorageComparer(
    $container->get('config.storage.sync'),
    $container->get('config.storage')
  );
  $storage_comparer->createChangelist();
  $drush_import->doImport($storage_comparer);

The first example does not work if you want to import new field storage definitions. This is what I've come up with:

$storage_comparer = new StorageComparer(
  \Drupal::service('config.storage.sync'),
  \Drupal::service('config.storage'),
  \Drupal::service('config.manager')
);
$config_importer = new ConfigImporter(
  $storage_comparer,
  \Drupal::service('event_dispatcher'),
  \Drupal::service('config.manager'),
  \Drupal::lock(),
  \Drupal::service('config.typed'),
  \Drupal::moduleHandler(),
  \Drupal::service('module_installer'),
  \Drupal::service('theme_handler'),
  \Drupal::service('string_translation')
);
$collection = StorageInterface::DEFAULT_COLLECTION;
$method = new ReflectionMethod($config_importer, 'processConfiguration');
$method->setAccessible(TRUE);
$method->invoke($config_importer, $collection, 'create', 'field.storage.node.field_subject');
$method->invoke($config_importer, $collection, 'create', 'field.field.node.mail.field_subject');
$context = [];
$config_importer->doSyncStep('finish', $context);

So, if you want to create fields it might be easier to just use Field(Storage)Config::create

When writing an update hook it is often necessary to force a specific config item to be imported from the config/sync folder. Based on the OP, here is the helper function I use within the mymodule.install file. This should work in both D8 and D9.

use Drupal\Core\Site\Settings;
use Drupal\Core\Config\FileStorage;

/**
 * Helper function to import single config file.
 *
 * @param string $config_name
 *   The name of the config item to import from the config sync folder.
 */
function mymodule_import_single_config($config_name) {
  $config_path = Settings::get('config_sync_directory');
  $source = new FileStorage($config_path);
  $config_storage = \Drupal::service('config.storage');
  $config_storage->write($config_name, $source->read($config_name));
}

/**
 * Imports filter config so we can remove a related module.
 */
function p2_core_update_8001() {
  mymodule_import_single_config('filter.format.basic_html');
}

It is not a good idea to write module config into active config directly, for example, it will erase some needed attributes, e.g _core and uuid which may cause issues later. It will also remove all other modifications by other modules if they are not imported into config (sometimes it is not possible due to dependencies like installation profile modules). Extra caution should be taken when updating config entities like fields, especially when creating them that way - db tables may not be created, entity API should be used instead.

My general advice when using this - update the parts you need not the whole configuration and when possible, use config entity object to change stuff.

That's the point of the snippet above: it is reading the actual config file from the config/sync folder, which contains the _core and uuid values already. Updating parts of a configuration is usually more fraught with danger. Importing config like this during an update hook is only a last resort however.

Sorry, I didn't specify, I meant the original snippet it just imports from module config, which doesn't (shouldn't) contain these values.
I just wanted to say something to other developers who will find this through Google search, that the original code might break stuff.

@mike-potter I can't get your code to work: it seems the configs are not imported. Has anything changed since your post (Aug 19, 2020) that would account for that?
[update] The code works as suggested for simple configs e.g. filter.format.basic_html where I changed the name to 'BASIC HTML'. In my use case I'm trying to import a list of some 30+ configs. That's where my issue lies... maybe with unmet dependencies...