docs: def _extend_settings(settings, configurator_config, prefix=None):
      """
      Extend settings dictionary with content of yaml's  configurator key.
  
      .. note::
  
          This methods changes multilayered subkeys defined
          within **configurator** into dotted keys in settings dictionary:
  
          .. code-block:: yaml
  
              configurator:
                  sqlalchemy:
                      url: mysql://user:password@host/dbname
  
          will result in **sqlalchemy.url**: mysql://user:password@host/dbname
          key value in settings dictionary.
  
      :param dict settings: settings dictionary
      :param dict configurator_config: yml defined settings
      :param str prefix: prefix for settings dict key
      """
      for key in configurator_config:
          settings_key = '.'.join([prefix, key]) if prefix else key
  
          if hasattr(configurator_config[key], 'keys') and\
                  hasattr(configurator_config[key], '__getitem__'):
              _extend_settings(
                  settings, configurator_config[key], prefix=settings_key
              )
          else:
              settings[settings_key] = configurator_config[key]