How to translate a pose in rospy?

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I've tried the following, which adds offset = 0.4 to a marker position in the x-axis. This works to some extent because it does move the marker. However, I want to move the marker relation to the robot base_link. Does that mean I've misunderstood and I actually do need to transform between multiple frames? In RViz, it looks the offset is currently applied in the map frame as the marker does not stay in front of the robot.

def pub_marker(self, robot_pose):
    marker_publisher = rospy.Publisher('visualization_marker', Marker, queue_size=1, latch=True)
    marker = Marker()
    offset = 0.4
    marker.pose.position.x = robot_pose.pose.pose.position.x
    marker.pose.position.x += offset
    marker.pose.position.y = robot_pose.pose.pose.position.y
    marker.pose.position.z = 0
    marker.pose.orientation = robot_pose.pose.pose.orientation
    marker_publisher.publish(marker)

The pose of an Rviz markers is defined relative to the coordinate frame set during the markers initialization.
For instance here we are creating a marker whose pose is relative to the map frame:

marker = Marker()
marker.header.frame_id = "/map"

Therefore if you offset this marker by, say 0.4 in the x-axis, it will appear as offset with respect to the map. If you want to offset with respect to the robot base, set your markers id_frame equal to base_link. In this case the marker will be published at a fixed pose with respect to the robot, and change its position with respect to the robot when you add an offset. Is this what you are trying to accomplish?

Excellent! This worked great. Thanks a lot :)

I am glad this works for you!

Hope its ok to revisit this question after I marked it as answered. I actually need to have marker.header.frame_id set as /map so my markers display where I want them to. Given this constraint, is tf2 needed to transform the marker pose so that it is offset in the x-axis relative to the base_link but the x, y and z values would be specified relative to the map frame? I've tried this so far (as posted in this question) but the transformed_pose just ends up with the same pose.position.x as transform.translation.x.

transform = TransformStamped()
transform.transform.translation.x = 0.2
transformed_pose = tf2_geometry_msgs.do_transform_pose(pose, transform)