'Default cache policy.
This defers to _use_cache on the Model class.
Args:
key: Key instance.
Returns:
A bool or None.'

@staticmethod
def default_cache_policy(key):

    flag = None
    if (key is not None):
        modelclass = model.Model._kind_map.get(key.kind())
        if (modelclass is not None):
            policy = getattr(modelclass, '_use_cache', None)
            if (policy is not None):
                if isinstance(policy, bool):
                    flag = policy
                else:
                    flag = policy(key)
    return flag
