Why must rospy.init_node be after rospy.Publisher?

edit

Consider the following simple program

#!/usr/bin/env python  
import rospy
import std_msgs

if __name__ == "__main__":
        pub = rospy.Publisher('/topic', std_msgs.msg.Int32, queue_size=1)
        rospy.init_node('test_py')
        pub.publish(0)

When I run rostopic echo /topic and run the node, it outputs 0 as expected.

However, when I swap the position of init_node and Publisher as so

#!/usr/bin/env python  
import rospy
import std_msgs

if __name__ == "__main__":
        rospy.init_node('test_py')
        pub = rospy.Publisher('/topic', std_msgs.msg.Int32, queue_size=1)
        pub.publish(0)

It is no longer able to publish.

Why is that the case? Given it would seem more natural (and in line with roscpp) to initialize the node via init_node before setting up publishers and subscribers.