Iterator from a file of follower information and return a tuple of screen_name, follower ids.
    File format is:
    <iso timestamp> <screen_name> <follower_id1> <follower_ids2> ...

def iter_follower_file(fname):
    """ Iterator from a file of follower information and return a tuple of screen_name, follower ids.
    File format is:
    <iso timestamp> <screen_name> <follower_id1> <follower_ids2> ...
    """
    with open(fname, 'rt') as f:
        for line in f:
            parts = line.split()
            if len(parts) > 3:
                yield parts[1].lower(), set(int(x) for x in parts[2:])