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You should really be using classes. But here is a hack that works

#!/usr/bin/env python
import rospy
from geometry_msgs.msg import PoseWithCovarianceStamped
from geometry_msgs.msg import TransformStamped

pub = rospy.Publisher('pose_after', TransformStamped, queue_size=40)
pose_after = TransformStamped()

def callback(data):

    pose_after.header.stamp = rospy.Time.now()
    pose_after.child_frame_id = 'base_footprint'

    pose_after.transform.translation.x = data.pose.pose.position.x
    pose_after.transform.translation.y = data.pose.pose.position.y
    pose_after.transform.translation.z = data.pose.pose.position.z
    pose_after.transform.rotation.x = data.pose.pose.orientation.x
    pose_after.transform.rotation.y = data.pose.pose.orientation.y
    pose_after.transform.rotation.z = data.pose.pose.orientation.z
    pose_after.transform.rotation.w = data.pose.pose.orientation.w

    print(pose_after.transform) # this prints (0,0)
    pub.publish(pose_after)

def listener():

    rospy.init_node('translater', anonymous=True)
    rospy.loginfo("Publishing pose_after")

    rospy.Subscriber('pose_before', PoseWithCovarianceStamped, callback)
    rate = rospy.Rate(40)

    while not rospy.is_shutdown():
        rate.sleep()

if __name__ == '__main__':
    try:
        listener()
    except rospy.ROSInterruptException:
        pass

In your original callback, you are trying to populate TransformStamped() which is a msg type, not a msg. you should be able to echo the pose_after topic and listen in on the new msgs now.