Bad message_filters.Subscriber callback: 'zip' object is not subscriptable

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Hi there,

I am having trouble with the message_filters.ApproximateTimeSynchronizer. I am working on a project where I need to process camera data and BoundingBoxes as seperate topics, but keep getting this weird error when I use the ApproximateTimeSynchronizer. Therefore I tried to set up a really simple code to test the time synchronisation, but it still fails. I don't know what's going on nor can I find useful information on the internet.

This is my code:

#!/usr/bin/env python3
#-*- coding:utf-8 -*-

import rospy
from std_msgs.msg import String, Int8

import message_filters
from message_filters import ApproximateTimeSynchronizer, Subscriber

class Example():

    def __init__(self):
        self.sub = message_filters.Subscriber('/chatter', Int8)
        self.sub2 = message_filters.Subscriber('/chatter2', Int8)
        self.ts = message_filters.ApproximateTimeSynchronizer([self.sub, self.sub2], 10, 0.3, allow_headerless=True)
        self.ts.registerCallback(self.synchron_fkt)

        self.pub = rospy.Publisher('/result_list', String, queue_size=10)
        rospy.loginfo("Launched node heading classification")
        rospy.spin()

    def synchron_fkt(self, chatter, chatter2):     
        result = f"the numbers are {chatter} and {chatter2}"
        rospy.loginfo(result)

if __name__ == '__main__':
    rospy.init_node('example', anonymous=True)
    hc = Example()

There are only random integers being published on the topics "/chatter" and "/chatter2". I can process the individual topics, when I call a individual callback function like

self.sub.registerCallback(self.callback_fkt)

So the topics are live, I can subscribe to them individually, but not with message_filters.ApproximateTimeSynchronizer. Here is the error I get:

[ERROR] [1643113118.568710]: bad callback: <bound method Subscriber.callback of <message_filters.Subscriber object at 0x7f2ae3f93f10>>
Traceback (most recent call last):
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rospy/topics.py", line 750, in _invoke_callback
    cb(msg)
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/message_filters/__init__.py", line 75, in callback
    self.signalMessage(msg)
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/message_filters/__init__.py", line 57, in signalMessage
    cb(*(msg + args))
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/message_filters/__init__.py", line 282, in add
    for vv in itertools.product(*[zip(*s)[0] for s in stamps]):
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/message_filters/__init__.py", line 282, in <listcomp>
    for vv in itertools.product(*[zip(*s)[0] for s in stamps]):
TypeError: 'zip' object is not subscriptable

I am using Ubuntu 16.04 LTS and ROS Kinetic with rosversion 1.12.17 . What does this TypeError mean? Is there any other way to process two seperate topics without using this message_filters?

When I use #!/usr/bin/env python2 at the start of the script, it works fine, but I need python3 to use pytorch and other packages in my bigger project. In my bigger project I am also using the python3 path, but I get the same error like above, only with the python3 path. Does anyone have an idea?

Thank you