How can I publish exactly once when the node is run?

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I want to publish a single /initialpose message when a node is run. How can I make sure it's only published once?

This is what I have:

#!/usr/bin/env python

import rospy
from geometry_msgs.msg import Pose, PoseWithCovarianceStamped
from std_msgs.msg import Header
from numpy import matrix

class PoseChange:
    def __init__(self):
        self.msg = PoseWithCovarianceStamped()
        rospy.init_node("pose_change", anonymous=True)
        rospy.Subscriber('/pose', Pose, self.callback)
        self.pub = rospy.Publisher('/initialpose', PoseWithCovarianceStamped, queue_size=3)

    def callback(self, data):
        self.msg.header = Header()
        self.msg.header.stamp = rospy.Time.now()
        self.msg.header.frame_id = "map"
        self.msg.pose.pose = data
        self.msg.pose.covariance = matrix([[0.09, 0, 0, 0, 0, 0], 
                                           [0, 0.09, 0, 0, 0, 0],
                                           [0, 0, 0.09, 0, 0, 0],
                                           [0, 0, 0, 0.25, 0, 0],
                                           [0, 0, 0, 0, 0.25, 0],
                                           [0, 0, 0, 0, 0, 0.25]])

    def run(self):
        self.pub.publish(self.msg)

if __name__ == "__main__":
    try:
        m = PoseChange()
        m.run()
    except rospy.ROSInterruptException:
        raise e

However, this never publishes when run. The /pose topic it's listening to is continuously publishing at 50 Hz.