quaternions with python

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I have recently started using rospy after being working with roscpp. I wanted to reproduce this piece of C++ code:

geometry_msgs::Quaternion quat = tf::createQuaternionMsgFromYaw(th);
geometry_msgs::PoseStamped goal;
goal.pose.orientation = quat;

For what I have found, the closest form would be something like:

quat = quaternion_to_msg(tf.transformations.quaternion_from_euler(0, 0, th))
goal = PoseStamped()
goal.pose.orientation = quat

But this actually does not work because 'quaternion_from_euler' returns a four float array, instead of an object with {x, y, z, w} fields.

I have searched for some common function to do this easy transform, but I have not found it within the ros libraries. What I have seen is that many people make his own implementation:

def quaternion_to_msg(q):
  msg = Quaternion()
  msg.x = q[0]
  msg.y = q[1]
  msg.z = q[2]
  msg.w = q[3]
  return msg

goal.pose.orientation = quaternion_to_msg(quat)

Does ROS provide a common solution for this situation?