Look at the following code from the head of structure.py, icet v2.1
This module provides the Structure class.
from typing import List, Sequence
from ase import Atom, Atoms
from _icet import _Structure
from .lattice_site import LatticeSite
_icet is a library written in c++ that includes the definition of class Structure rather than _Structure. If you try simply by “from _icet import _Structure”. python interpreter just reports error since it cannot find where _Structure is.