Problem with Subscriber class from message_filter in Python

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I am currently facing a problem, that the Subscriber() is expecting 3 arguments, while in the docs it is always being done with 2: http://docs.ros.org/en/api/message_fi...

Therefore I am getting an IndexError: tuple index out of range. Looking at the source code of Subscriber:

class Subscriber(SimpleFilter):

"""
ROS2 subscription filter,Identical arguments as :class:`rclpy.Subscriber`.

This class acts as a highest-level filter, simply passing messages
from a ROS2 subscription through to the filters which have connected
to it.
"""

def __init__(self, *args, **kwargs):
    SimpleFilter.__init__(self)
    self.node = args[0]
    self.topic = args[2]
    kwargs.setdefault('qos_profile', 10)
    self.sub = self.node.create_subscription(*args[1:], self.callback, **kwargs)

def callback(self, msg):
    self.signalMessage(msg)

def getTopic(self):
    return self.topic

def __getattr__(self, key):
    """Serve same API as rospy.Subscriber"""
    return self.sub.__getattribute__(key)

you can see that let's say

imu_sub = Subscriber("imu/", Imu)

won't work, as it expects 3 arguments. As I consider myself still as a novice in Python and ROS2, I'd appreciate any help on this topic.