Disabling parallelism of subscribers in Python

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Hi all, I have observed that rospy is enabling the parallelism of subscriber callbacks which means that before one callback that receives a message from a topic finishes, another callback that receives a message from another topic can start execution. This might be ideal for most applications, but in my case it is not the desired behavior. To make it simple and general I am sharing the below code that would serve as an example:

#!/usr/bin/env python

import rospy
from std_msgs.msg import String

def callback1(msg):
    while(True):
        print("Callback1: %s" % (msg.data))
        rospy.sleep(1)

def callback2(msg):
    while(True):
        print("Callback2: %s" % (msg.data))
        rospy.sleep(1)

def main():
    rospy.init_node('foo')
    rospy.Subscriber('foo1', String, callback1, queue_size=1)
    rospy.Subscriber('foo2', String, callback2, queue_size=1)
    rospy.spin()

if __name__ == '__main__':
    main()

And the output is like:

Callback1: qwe
Callback2: asd
Callback1: qwe
Callback2: asd
Callback1: qwe

Is there any way to force a node to use a single thread for all of its callbacks which would mean a callback blocking the others from starting to execute until it finishes? Thanks in advance!