'Set the context datastore policy function.
Args:
func: A function that accepts a Key instance as argument and returns
a bool indicating if it should use the datastore.  May be None.'

def set_datastore_policy(self, func):

    if (func is None):
        func = self.default_datastore_policy
    elif isinstance(func, bool):
        func = (lambda unused_key, flag=func: flag)
    self._datastore_policy = func
