docs: def update_from_env(yaml_dict, prefix=None):
      '''
      Override YAML settings with values from the environment variables.
  
          - The letter '_' is delimit the hierarchy of the YAML settings such
            that the value of 'config.databases.local' will be overridden
            by CONFIG_DATABASES_LOCAL.
      '''
      prefix = prefix or ""
  
      def _set_env_var(path, node):
          env_path = "{0}{1}{2}".format(
              prefix.upper(),
              '_' if prefix else '',
              '_'.join([str(key).upper() for key in path])
          )
          env_val = os.environ.get(env_path, None)
          if env_val is not None:
              # convert the value to a YAML-defined type
              env_dict = yamldict.load('val: {0}'.format(env_val))
              return env_dict.val
          else:
              return None
  
      # traverse yaml_dict with the callback function
      yaml_dict.traverse(_set_env_var)