old_contents: import numpy as np
  import pytest
  
  import mbuild as mb
  from mbuild.utils.io import get_fn
  from mbuild.tests.base_test import BaseTest
  from mbuild.exceptions import MBuildError
  
  
  class TestXYZ(BaseTest):
      def test_load_no_top(self, ethane):
          ethane.save(filename='ethane.xyz')
          ethane_in = mb.load('ethane.xyz')
          assert len(ethane_in.children) == 8
          assert ethane_in.n_bonds == 0
          assert set([child.name for child in ethane_in.children]) == {'C', 'H'}
  
      def test_wrong_n_atoms(self):
          with pytest.raises(MBuildError):
              mb.load(get_fn('too_few_atoms.xyz'))
          with pytest.raises(MBuildError):
              mb.load(get_fn('too_many_atoms.xyz'))
  
      def test_save(self, ethane):
          ethane.save(filename='ethane.xyz')
          ethane_in = mb.load('ethane.xyz')
          assert len(ethane_in.children) == 8
          assert set([child.name for child in ethane_in.children]) == {'C', 'H'}
  
      def test_coordinates(self, ethane):
          ethane.save(filename='ethane.xyz')
          ethane_in = mb.load('ethane.xyz')
          assert np.allclose(ethane.xyz, ethane_in.xyz)
  
new_contents: import numpy as np
  import pytest
  
  import mbuild as mb
  from mbuild.formats.xyz import write_xyz
  from mbuild.utils.io import get_fn
  from mbuild.tests.base_test import BaseTest
  from mbuild.exceptions import MBuildError
  
  
  class TestXYZ(BaseTest):
      def test_load_no_top(self, ethane):
          ethane.save(filename='ethane.xyz')
          ethane_in = mb.load('ethane.xyz')
          assert len(ethane_in.children) == 8
          assert ethane_in.n_bonds == 0
          assert set([child.name for child in ethane_in.children]) == {'C', 'H'}
  
      def test_wrong_n_atoms(self):
          with pytest.raises(MBuildError):
              mb.load(get_fn('too_few_atoms.xyz'))
          with pytest.raises(MBuildError):
              mb.load(get_fn('too_many_atoms.xyz'))
  
      def test_bad_input(self, ethane):
          with pytest.raises(ValueError):
              assert isinstance(ethane, mb.Compound)
              write_xyz(ethane, 'compound.xyz')
  
      def test_save(self, ethane):
          ethane.save(filename='ethane.xyz')
          ethane_in = mb.load('ethane.xyz')
          assert len(ethane_in.children) == 8
          assert set([child.name for child in ethane_in.children]) == {'C', 'H'}
  
      def test_coordinates(self, ethane):
          ethane.save(filename='ethane.xyz')
          ethane_in = mb.load('ethane.xyz')
          assert np.allclose(ethane.xyz, ethane_in.xyz)
  
subject: Add test to ensure write_xyz does not directly take in compound