docs: def ensure_environment(variables):
      """
      Check os.environ to ensure that a given collection of
      variables has been set.
  
      :param variables: A collection of environment variable names
      :returns: os.environ
      :raises IncompleteEnvironment: if any variables are not set, with
          the exception's ``variables`` attribute populated with the
          missing variables
      """
      missing = [v for v in variables if v not in os.environ]
      if missing:
          formatted = ', '.join(missing)
          message = 'Environment variables not set: {}'.format(formatted)
          raise IncompleteEnvironment(message, missing)
      return os.environ