'Refresh an old attribute.
Note that refreshing the attribute is asynchronous. It does not change
anything about the current object - you need to reopen the same URN some
time later to get fresh data.
Attributes:
CONTAINS - Refresh the content of the directory listing.
Args:
attribute: An attribute object as listed above.
priority: Priority to set for updating flow, None for default.
Returns:
The Flow ID that is pending
Raises:
IOError: If there has been an error starting the flow.'

def Update(self, attribute=None, priority=None):

    client_id = self.urn.Split()[0]
    if (attribute == 'CONTAINS'):
        flow_id = flow.GRRFlow.StartFlow(client_id=client_id, flow_name='ListDirectory', pathspec=self.real_pathspec, priority=priority, notify_to_user=False, token=self.token)
        return flow_id
