Rotation Angle in Pose Orientation

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I have a 2D image on which I conduct an algorithm to find its rotation, and I get it in radian. No problem until here.

Now that I want to fill in the pose object with what I collected from the 2D vision, I get stuck at where to insert the rotation. I do know I cannot fill in everything, I don't intend to, either. I only need the position (I have it already) and the rotation.

            obj.pose.position.x = v['pose'][0] #x 
            obj.pose.position.y = v['pose'][1] #y 
            obj.pose.position.z = v['pose'][2] #z

             # which one below is the one I should use to pass the rotation angle?
            obj.pose.orientation.x = v['pose'][3] 
            obj.pose.orientation.y = v['pose'][4]
            obj.pose.orientation.z = v['pose'][5]
            obj.pose.orientation.w = v['pose'][6]

Any thoughts?

EDIT: So I came up with the following after the useful insights given by the commentators.

       self.some_srv = rospy.Service("/bla/request_poses", PartArray, self.some_service_srv)

       def some_service_srv(self, req):
         # some irrelevant stuff happening here

        # this is where the actual thing shall happen
        obj.pose.position.x = v['pose'][0]
        obj.pose.position.y = v['pose'][1]
        obj.pose.position.z = v['pose'][2]
        rotation_angle = v['pose'][3] # extract the rotation angle
        zaxis = (0, 0, 1) #rotation is around the Z axis in the image
        quaternion = quaternion_about_axis(rotation_angle, zaxis) # use rotation angle + axis to get the quaternion
        obj.pose.orientation.x = quaternion[0]
        obj.pose.orientation.y = quaternion[1]
        obj.pose.orientation.z = quaternion[2] # this is the rotation we are talking about
        obj.pose.orientation.w = quaternion[3]

        list_detected.part_array.append(obj)
        return list_detected

This runs, however I always get 0 values for all of the quarternions, which is somehow nonsense because the rotation angle is definitely non-zero (I print its value).

            position: 
              x: 274.0
              y: 250.0
              z: 602.0
            orientation: 
              x: 0.0
              y: 0.0
              z: 0.0
              w: 0.0

I print the quarternion list separately, and there is a non-zero value inside, which seems fine, but I don't know why the published value gets zero.

         [0.000000e+00 0.000000e+00 3.061617e-17 1.000000e+00]

I also printed the value of rotation (in radian) and here are some values of it:

0.980580687945
6.12323399574e-17
-0.707106781187
0.294085862655
-1.0
0.56419054038
-0.0114934175734
6.12323399574e-17
6.12323399574e-17
-1.0
0.707106781187
6.12323399574e-17

And this is how I get the angle using OpenCV:

# helper function to find out which side of the rectangle is longer, then add the angle appropriately
def getAngle(rect):
    angle = rect[2]
    width, height = rect[1]
    if (width < height):
        return math.cos(math.radians(angle + 180))
    else:
        return math.cos(math.radians(angle + 90))



...
 # Find contours:
    (im, contours, hierarchy) = cv2.findContours(im, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
    cnt = contours[0]

    # get the rotated rectangle to get the rotation
    rect = cv2.minAreaRect(cnt)
    rotation = getAngle(rect) # in radians
...