import bpy

from mathutils import *
from math import *

def compute_rotation(src_vec, dest_vec):
    """ return an orientation quaternion q such that
    dest_vec = c * q * src_vec
    (where c = dest_vec.magnitude / src_vec.magnitude and is not interesting in terms of orientation)"""
    a = src_vec.normalized()
    b = dest_vec.normalized()

    c = a.cross(b)

    theta = asin(c.magnitude)

    if (c.magnitude>0):
        axis = c.normalized()
        st2 = sin(theta/2)
        q = Quaternion( [cos(theta/2), st2*axis.x, st2*axis.y, st2*axis.z] )
    else:
        q = Quaternion( [1,0,0,0] )

    return q

obj = bpy.context.active_object

obj.rotation_mode = 'QUATERNION'
obj.rotation_quaternion = compute_rotation(Vector([1,1,1]), Vector([1,0,0]))