code: def _find_batch_containing_event(self, uuid):
          
          if self.estore.key_exists(uuid):
              # Reusing already opened DB if possible
              return self.batchno
          else:
              for batchno in range(self.batchno - 1, -1, -1):
                  # Iterating backwards here because we are more likely to find
                  # the event in an later archive, than earlier.
                  db = self._open_event_store(batchno)
                  with contextlib.closing(db):
                      if db.key_exists(uuid):
                          return batchno
          return None
docs: Find the batch number that contains a certain event.
  
          Parameters:
          uuid    -- the event uuid to search for.
  
          returns -- a batch number, or None if not found.