<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _GetRoutingMap(self, router):
 """Returns a routing map for a given router instance."""


    try:
      routing_map = self._routing_maps_cache.Get(router.__class__)
    except KeyError:
      routing_map = self._BuildHttpRoutingMap(router.__class__)
      self._routing_maps_cache.Put(router.__class__, routing_map)

    return routing_map