code: def _load_serialized_parts(phys_reader, pkg_srels, content_types):
          
          sparts = []
          part_walker = PackageReader._walk_phys_parts(phys_reader, pkg_srels)
          for partname, blob, srels in part_walker:
              content_type = content_types[partname]
              spart = _SerializedPart(partname, content_type, blob, srels)
              sparts.append(spart)
          return tuple(sparts)
docs: Return a list of |_SerializedPart| instances corresponding to the
          parts in *phys_reader* accessible by walking the relationship graph
          starting with *pkg_srels*.