How to use the chempy.cpv.length function in chempy

def pdbBox(line):
        ''' Parses a gro box line and returns a pdb CRYST1 line '''
        from math import degrees
        from chempy.cpv import length, get_angle

        v = [10*float(i) for i in line.split()] + 6*[0] # Padding for rectangular boxes
        v1, v2, v3 = (v[0], v[3], v[4]), (v[5], v[1], v[6]), (v[7], v[8], v[2])

        a = length(v1)
        b = length(v2)
        c = length(v3)

        alpha = degrees(get_angle(v2, v3))
        beta = degrees(get_angle(v1, v3))
        gamma = degrees(get_angle(v1, v2))

        return _pdbBoxLine % (a, b, c, alpha, beta, gamma)

0.8, 0.8, 0.8,
    ])
    obj.extend(color_list)

    if symmetric:
        start = cpv.sub(center, cpv.scale(direction, scale))
        obj.append(cgo.SAUSAGE)
        obj.extend(center)
        obj.extend(start)
        obj.extend([
            radius,
            0.8, 0.8, 0.8,
        ])
        obj.extend(color2_list)

    coneend = cpv.add(end, cpv.scale(direction, 4.0*radius/cpv.length(direction)))
    obj.append(cgo.CONE)
    obj.extend(end)
    obj.extend(coneend)
    obj.extend([
        radius * 1.75,
        0.0,
    ])
    obj.extend(color_list * 2)
    obj.extend([
        1.0, 1.0, # Caps
    ])
    cmd.load_cgo(obj, get_unused_name('oriVec'), zoom=0)

SEE ALSO

    helix_orientation
    '''
    visualize, quiet, cutoff = int(visualize), int(quiet), float(cutoff)
    stored.x = dict()
    cmd.iterate_state(STATE, '(%s) and name N+O' % (selection),
                      'stored.x.setdefault(resv, dict())[name] = x,y,z')
    vec_list = []
    for resi in stored.x:
        resi_other = resi + 4
        if 'O' in stored.x[resi] and resi_other in stored.x:
            if 'N' in stored.x[resi_other]:
                vec = cpv.sub(stored.x[resi_other]['N'], stored.x[resi]['O'])
                if cpv.length(vec) < cutoff:
                    vec_list.append(vec)
    if len(vec_list) == 0:
        print('warning: count == 0')
        raise CmdException
    vec = _vec_sum(vec_list)
    vec = cpv.normalize(vec)
    return _common_orientation(selection, vec, visualize, quiet)

SEE ALSO
 
    helix_orientation
    '''
    visualize, quiet, cutoff = int(visualize), int(quiet), float(cutoff)
    stored.x = dict()
    cmd.iterate_state(-1, '(%s) and name N+O' % (selection),
            'stored.x.setdefault(resv, dict())[name] = x,y,z')
    vec_list = []
    for resi in stored.x:
        resi_other = resi + 4
        if 'O' in stored.x[resi] and resi_other in stored.x:
            if 'N' in stored.x[resi_other]:
                vec = cpv.sub(stored.x[resi_other]['N'], stored.x[resi]['O'])
                if cpv.length(vec) < cutoff:
                    vec_list.append(vec)
    if len(vec_list) == 0:
        print 'warning: count == 0'
        raise CmdException
    vec = _vec_sum(vec_list)
    vec = cpv.normalize(vec)
    return _common_orientation(selection, vec, visualize, quiet)

center2 = centerofmass(selection2)
        print(' Angle: %.2f deg, Displacement: %.2f angstrom' % (angle, cpv.distance(center1, center2)))

        if visualize:
            center1 = numpy.array(center1, float)
            center2 = numpy.array(center2, float)
            center = (center1 + center2) / 2.0

            if gyradius is not None:
                rg = numpy.array(gyradius(selection1), float)
            else:
                rg = 10.0

            h1 = numpy.cross(center2 - center1, direction)
            h2 = numpy.dot(R33, h1)
            h1 *= rg / cpv.length(h1)
            h2 *= rg / cpv.length(h2)

            for pos in [center1, center2, center1 + h1, center1 + h2]:
                cmd.pseudoatom(mobile_tmp, pos=list(pos), state=1)

            # measurement object for angle and displacement
            name = get_unused_name('measurement')
            cmd.distance(name, *['%s`%d' % (mobile_tmp, i) for i in [1,2]])
            cmd.angle(name, *['%s`%d' % (mobile_tmp, i) for i in [3,1,4]])

            # CGO arrow for axis of rotation
            visualize_orientation(direction, center1, rg, color='blue')

            cmd.delete(mobile_tmp)

    return angle