rospy.sleep doesn't get awaken

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Only on one of my machine, this elementary subscribe/publishing tutorial (WritingPublisherSubscriber(python)) doesn't work. In talker.py, thread never returns from rospy.sleep(t) (t can be double number). On my other machine this doesn't happen. Any idea?

Environment:

• where problem occurs: Ubuntu 11.04, electric, Python 2.7.1+
• where does not: Ubuntu 10.10, electric, Python 2.6.6

Looking at these diff, I just tried to use python 2.6.6 (by downgrading like this), and found no effect.

Source code (exactly the same with the one in the tutorial page):

#!/usr/bin/env python
import roslib; roslib.load_manifest('beginner_tutorials')
import rospy
from std_msgs.msg import String
def talker():
    pub = rospy.Publisher('chatter', String)
    rospy.init_node('talker')
    while not rospy.is_shutdown():
        str = "hello world %s"%rospy.get_time()
        rospy.loginfo(str)
        pub.publish(String(str))
        rospy.sleep(1.0)
if __name__ == '__main__':
    try:
        talker()
    except rospy.ROSInterruptException: pass

Update) It also occurred when I use C++ version of the same tutorial.