code: def get_subnet(context, id, fields=None):
      
      LOG.info("get_subnet %s for tenant %s with fields %s" %
               (id, context.tenant_id, fields))
      subnet = db_api.subnet_find(context=context, limit=None,
                                  page_reverse=False, sorts=['id'],
                                  marker_obj=None, fields=None, id=id,
                                  join_dns=True, join_routes=True,
                                  scope=db_api.ONE)
      if not subnet:
          raise n_exc.SubnetNotFound(subnet_id=id)
  
      cache = subnet.get("_allocation_pool_cache")
      if not cache:
          new_cache = subnet.allocation_pools
          db_api.subnet_update_set_alloc_pool_cache(context, subnet, new_cache)
      return v._make_subnet_dict(subnet)
docs: Retrieve a subnet.
  
      : param context: neutron api request context
      : param id: UUID representing the subnet to fetch.
      : param fields: a list of strings that are valid keys in a
          subnet dictionary as listed in the RESOURCE_ATTRIBUTE_MAP
          object in neutron/api/v2/attributes.py. Only these fields
          will be returned.