Infinite loop in callback function

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Hi there! I want to subscribe to a topic (real_position) and save these data in a variable (julio) in order to process them later in my code. I've tried to use something similar to the tutorial for the subscriber node: http://wiki.ros.org/ROS/Tutorials/Wri... The problem is that when I run my node, the code stacks in the callback function, repeating it again and again, until I press Ctrl + C. Then it continues with the rest of the code. Is callback function the only way to get subscribed data? and if it is, how can I make it run once per loop and get the data to a variable? I am new to ROS and Python, and I apologize if this sounds trivial. I use Ubuntu 16.04 LTS, Python 2.7.12 and ROS Kinetic. Thank you in advance!

#!/usr/bin/python
import rospy
import serial
import time
from std_msgs.msg import String

rospy.init_node('serial_connection', anonymous = True)

ser = serial.Serial('/dev/ttyUSB0', 9600)
print (ser.name)
time.sleep(2)

x_d = 0 # mm
y_d = 1000
thita_d = 0 # moires i rads

camera = [0, 0, 0]
error = [0, 0, 0]
sending_data = [0, 0, 0]
print(sending_data)

def callback(data):
    print("callback")
    global camera
    camera = data
    print(camera)
    print("emeina mes to callback kollimeno")

rospy.Subscriber('real_position', String, callback)
rate = rospy.Rate(1) #isws na min xreiazetai


def camera_feedback():
    print("camera feedback")
    print(camera)

def error_calculation():
    print("error caclulation")
    error[0] = x_d - camera[0]
    error[1] = y_d - camera[1]
    error[2] = thita_d - camera[2]

def controller_equations():
    print("controller equations")
    if error != [0, 0, 0]:
        print(error)
        global sending_data
        print(sending_data)
        sending_data[0] = 8
        sending_data[1] = 8
        sending_data[2] = 0

    else:
        sending_data = [0, 0, 0]
    print(error)
    print('kokos')
    print(sending_data)


def serial_connection():
    print("serial connection")
    if not rospy.is_shutdown():      
        if ser.isOpen():
            print ("Port Open")
            for i in range(1):               
                ser.write(bytes(sending_data) + '\n')
                time.sleep(0.001)




if __name__ == '__main__':
    try:
        while not rospy.is_shutdown():
            print("in the while loop")
            camera_feedback()
            error_calculation()
            controller_equations()
            serial_connection()
            rate.sleep()
        ser.close()
    except rospy.ROSInterruptException:
        pass