class _Primary(object):

    @property
    def mongos_mode(self):
        return 'primary'

    @property
    def mode(self):
        return 0

    @property
    def name(self):
        return 'Primary'

    @property
    def document(self):
        return {'mode': 'primary'}

    @property
    def tag_sets(self):
        return [{}]

    @property
    def max_staleness(self):
        return -1

    @property
    def min_wire_version(self):
        return 0


def ensure_read_preference_type(key, value):
    """Raise a TypeError if the value is not a type compatible for ReadPreference."""
    for attr in ('document', 'mode', 'mongos_mode', 'max_staleness'):
        if not hasattr(value, attr):
            raise TypeError('{} must be an instance of {}'.format(
                key, 'pymongo.read_preference.ReadPreference'))


PRIMARY = _Primary()