How to get number of messages in one topic from rosbag.py

Hello! I was wondering whether with the Python module rosbag there is any chance to get the number of recorded messages in one topic. The Bag.read_messages method returns a generator and of course I could iterate over all messages and count. But is there a way to tell the number in advance? That way I could preallocate the right amount of lists to handle the messages. Thank you!