def test_setpos():
    import numpy as np
    from ase.build import molecule
    from ase.constraints import FixAtoms

    def array_almost_equal(a1, a2, tol=np.finfo(type(1.0)).eps):
        """Replacement for old numpy.testing.utils.array_almost_equal."""
        return (np.abs(a1 - a2) < tol).all()

    m = molecule('H2')
    c = FixAtoms(indices=[atom.index for atom in m])
    m.set_constraint(c)

    pos1 = m.get_positions()
    # shift z-coordinates by 1.
    pos = m.get_positions()
    pos[:, 2] += 1.

    m.set_positions(pos)
    # note that set_positions fails silently to set the new positions
    # due to the presence of constraints!
    assert array_almost_equal(pos1, m.get_positions())

    m.positions = pos
    # atoms.positions allows one to set the new positions
    # even in the presence of constraints!
    assert array_almost_equal(pos, m.get_positions())