How to use the cachier.mongo_core._MongoCore function in cachier

def test_stalled_mongo_db_cache():
    @cachier(mongetter=_test_mongetter)
    def _stalled_func():
        return 1
    core = _MongoCore(_test_mongetter, None, False)
    core.set_func(_stalled_func)
    core.clear_cache()
    with pytest.raises(RecalculationNeeded):
        core.wait_on_entry_calc(key=None)

pickle_reload (optional) : bool
        If set to True, in-memory cache will be reloaded on each cache read,
        enabling different threads to share cache. Should be set to False for
        faster reads in single-thread programs. Defaults to True.
    mongetter (optional) : callable
        A callable that takes no arguments and returns a pymongo.Collection
        object with writing permissions. If unset a local pickle cache is used
        instead.
    """
    # print('Inside the wrapper maker')
    # print('mongetter={}'.format(mongetter))
    # print('stale_after={}'.format(stale_after))
    # print('next_time={}'.format(next_time))

    if mongetter:
        core = _MongoCore(mongetter, stale_after, next_time)
    else:
        core = _PickleCore(  # pylint: disable=R0204
            stale_after, next_time, pickle_reload)

    def _cachier_decorator(func):
        core.set_func(func)

        @wraps(func)
        def func_wrapper(*args, **kwds):  # pylint: disable=C0111,R0911
            # print('Inside general wrapper for {}.'.format(func.__name__))
            ignore_cache = kwds.pop('ignore_cache', False)
            overwrite_cache = kwds.pop('overwrite_cache', False)
            verbose_cache = kwds.pop('verbose_cache', False)
            _print = lambda x: None
            if verbose_cache:
                _print = print

def get_entry_by_key(self, key):
        res = self.mongo_collection.find_one({
            'func': _MongoCore._get_func_str(self.func),
            'key': key
        })
        if res:
            try:
                entry = {
                    'value': pickle.loads(res['value']),
                    'time': res.get('time', None),
                    'stale': res.get('stale', False),
                    'being_calculated': res.get('being_calculated', False)
                }
            except KeyError:
                entry = {
                    'value': None,
                    'time': res.get('time', None),
                    'stale': res.get('stale', False),
                    'being_calculated': res.get('being_calculated', False)