def test_custom_subsystem(self):
lattice = Lattice([8, 8], 2)
subsystem = CustomSubsystem(lambda site: site.bs[0] % 2 == 0, lattice)
inside = set()
for site in lattice:
if site in subsystem:
inside.add(site)
else:
with pytest.raises(ValueError):
subsystem.index(site)
assert len(inside) == len(subsystem)
for i, site in enumerate(subsystem):
assert subsystem[i] == site
assert subsystem.index(site) == i
inside.remove(site)
assert len(inside) == 0
