def test_fixbonds():
    """Test Atoms.__delitem__ with FixAtoms constraint."""
    from ase import Atoms
    from ase.constraints import FixBondLengths

    a = Atoms('H3')
    a.constraints = FixBondLengths([(1, 2)])
    assert (a[:].constraints[0].pairs == [(1, 2)]).all()
    assert (a[1:].constraints[0].pairs == [(0, 1)]).all()
    assert len(a[2:].constraints) == 0
    assert len(a[1:2].constraints) == 0
    assert len(a[:2].constraints) == 0
    assert len(a[:1].constraints) == 0

    # Execise Atoms.__init__:
    Atoms(a)