I looked at this: http://answers.ros.org/question/207071/how-to-fill-up-a-pointcloud-message-with-data-in-python/

And tried the following:

Point cloud

    h = std_msgs.msg.Header()
    h.stamp = rospy.Time.now()
    pcl = PointCloud()
    pcl.header = h
    pcl.points = self.Npop_particles
    pcl.channels = 3
    point = Point()

    print pcl.points

    for i in range(0, self.Npop_particles):
        point.x = 0
        point.y = 0
        point.z = 0
        pcl.points[i].x = point.x
        pcl.points[i].y = point.y
        pcl.points[i].z = point.z

But I get an error saying pcl.points[i] is an integer:

print pcl.points[i].x TypeError: 'int' object has no attribute 'getitem'