@property
def active_device(cls):
    """The device used by PYNQ if `None` used for a device parameter

        This defaults to the device with the lowest priority and index but
        can be overridden to globally change the default.

        """
    if not hasattr(cls, '_active_device'):
        if len(cls.devices) == 0:
            raise RuntimeError('No Devices Found')
        cls._active_device = cls.devices[0]
    return cls._active_device
