Update the global variable in Rospy

Question

i have the problem. I have two node, node 1 and node2, where node1 send a float number in node2 and node2 send a float number in node1. In the function callback, I want sum the information received with other value and update the variable. But the problem is that i don't succeed the information sent, because the terminal prints in output only first update(for node 1 i get 2.9, and node 2 2.0) These code for node1 and node2

Node1
    !/usr/bin/env python
    import rospy
    import time
    from std_msgs.msg import Float64 
    global x1
    global a 
    x1 = 1.5
    def callback(msg):
        #print 'Sto ricevendo informazioni da %s nel tempo %s' % (msg.data, time.ctime())
         #print "%f"%msg.data
        a = msg.data  
        info_nodo2 = a + 0.5
        x1 = info_nodo2
        print "%f"%x1  
    def nodo():
        pub = rospy.Publisher('chatter1', Float64)
        rospy.init_node('nodo1', anonymous=True)
        rospy.loginfo("In attesa")
        rospy.Subscriber('chatter2', Float64, callback)
        rate = rospy.Rate(1) # 10hz
        while not rospy.is_shutdown():
            for i in range(1,51):   
                #rospy.loginfo(num)
                pub.publish(x1)
                rate.sleep()
            rospy.spin()
    if __name__ == '__main__':
        try:
            nodo()
        except rospy.ROSInterruptException:
            pass

 Node2

    import rospy
    import time
    from std_msgs.msg import Float64
    global x2 
    global a 
    x2 = 2.4
    def callback(msg):
        #print 'Sto ricevendo informazioni da %s nel tempo %s' % (msg.data, time.ctime())
         #print "%f"%msg.data     
        a = msg.data  
        info_nodo1 = a + 0.5
        x2 = info_nodo1
        print "%f"%x2
    def nodo():
        pub = rospy.Publisher('chatter2', Float64)
        rospy.init_node('nodo2', anonymous=True)
        rospy.loginfo("In attesa")
        rospy.Subscriber('chatter1', Float64, callback) 
        rate = rospy.Rate(1) # 10hz   
        while not rospy.is_shutdown():
            for i in range(1,51):
               # num = "%s" % (x2)
                #rospy.loginfo(num)
                pub.publish(x2)
                rate.sleep()
            rospy.spin() 
    if __name__ == '__main__':
        try:
            nodo()
        except rospy.ROSInterruptException:
            pass

Answer 1

The problem is you are declaring x1 and x2 as global variables. At every iteration their values get reset to 1.5 and 2.4, respectively. Thus, the published values are those values plus the 0.5 (i.e 2.0 and 2.9).

If I understand correctly, you want the two nodes to continuously update each others values (for x1 and x2). I wrote your nodes as classes instead and replaced the global variables by instance variables:

Node1:

#!/usr/bin/env python
# -*- encoding: utf-8 -*-
import rospy
import std_msgs.msg


class Nodo(object):
    def __init__(self):
        # Params
        self.x1 = 1.5
        self.a = None

        # Node cycle rate (in Hz).
        self.loop_rate = rospy.Rate(10)

        # Publishers
        self.pub = rospy.Publisher("~chatter1", std_msgs.msg.Float64, queue_size=10)

        # Subscribers
        rospy.Subscriber("~chatter2", std_msgs.msg.Float64, self.callback)

    def callback(self, msg):
        self.a = msg.data
        self.x1 = self.a + 0.5
        rospy.loginfo("x1: {}".format(self.x1))

    def start(self):
        rospy.loginfo("In attesa")

        while not rospy.is_shutdown():
            for ii in xrange(1, 51):
                self.pub.publish(self.x1)
                self.loop_rate.sleep()

if __name__ == '__main__':
    rospy.init_node("nodo1", anonymous=True)
    my_node = Nodo()
    my_node.start()

Node2:

#!/usr/bin/env python
# -*- encoding: utf-8 -*-
import rospy
import std_msgs.msg


class Nodo(object):
    def __init__(self):
        # Params
        self.x2 = 2.4
        self.a = None

        # Node cycle rate (in Hz).
        self.loop_rate = rospy.Rate(10)

        # Publishers
        self.pub = rospy.Publisher("~chatter2", std_msgs.msg.Float64, queue_size=10)

        # Subscribers
        rospy.Subscriber("~chatter1", std_msgs.msg.Float64, self.callback)

    def callback(self, msg):
        self.a = msg.data
        self.x2 = self.a + 0.5
        rospy.loginfo("x2: {}".format(self.x2))

    def start(self):
        rospy.loginfo("In attesa")

        while not rospy.is_shutdown():
            for ii in xrange(1, 51):
                self.pub.publish(self.x2)
                self.loop_rate.sleep()

if __name__ == '__main__':
    rospy.init_node("nodo2", anonymous=True)
    my_node = Nodo()
    my_node.start()

When running these two nodes the console output is as follows:

process[nodo_1-1]: started with pid [7688]
process[nodo_2-2]: started with pid [7689]
[INFO] [WallTime: 1478865725.627418] In attesa
[INFO] [WallTime: 1478865725.627904] In attesa
[INFO] [WallTime: 1478865725.725064] x2: 2.0
[INFO] [WallTime: 1478865725.725512] x1: 2.5
[INFO] [WallTime: 1478865725.825050] x2: 3.0
[INFO] [WallTime: 1478865725.825448] x1: 3.5
[INFO] [WallTime: 1478865725.925056] x2: 4.0
[INFO] [WallTime: 1478865725.925608] x1: 4.5
[INFO] [WallTime: 1478865726.025061] x2: 5.0
[INFO] [WallTime: 1478865726.025617] x1: 5.5
[INFO] [WallTime: 1478865726.125045] x2: 6.0
[INFO] [WallTime: 1478865726.125605] x1: 6.5
[INFO] [WallTime: 1478865726.225033] x2: 7.0
[INFO] [WallTime: 1478865726.225586] x1: 7.5
[INFO] [WallTime: 1478865726.325013] x2: 8.0
[INFO] [WallTime: 1478865726.325606] x1: 8.5
[INFO] [WallTime: 1478865726.425041] x2: 9.0
[INFO] [WallTime: 1478865726.425608] x1: 9.5
[INFO] [WallTime: 1478865726.525057] x2: 10.0
[INFO] [WallTime: 1478865726.525545] x1: 10.5
[INFO] [WallTime: 1478865726.625054] x2: 11.0

Hope this helps.