code: def is_running(self):
          
          if self._postmaster_proc:
              if self._postmaster_proc.is_running():
                  return self._postmaster_proc
              self._postmaster_proc = None
  
          # we noticed that postgres was restarted, force syncing of replication
          self._schedule_load_slots = self.use_slots
  
          self._postmaster_proc = PostmasterProcess.from_pidfile(self._data_dir)
          return self._postmaster_proc
docs: Returns PostmasterProcess if one is running on the data directory or None. If most recently seen process
          is running updates the cached process based on pid file.