def priority(cls):
    """
        Each backend class must supply a priority, a number (float or integer)
        indicating the priority of the backend relative to all other backends.
        The priority need not be static -- it may (and should) vary based
        attributes of the environment in which is runs (platform, available
        packages, etc.).

        A higher number indicates a higher priority. The priority should raise
        a RuntimeError with a message indicating the underlying cause if the
        backend is not suitable for the current environment.

        As a rule of thumb, a priority between zero but less than one is
        suitable, but a priority of one or greater is recommended.
        """
