def get_fastqs_for_read_index(lane_to_fastqs, read_index):
    """Get a list of fastq urls for the given read index

    Args:
        lane_to_fastqs (Dict[Dict[str, str]]): dict of dict mapping each lane to a dict of read index to fastq url
        read_index (str): the read_index to filter by, e.g. "read1", "read2", or "index1"

    Returns:
        fastq_urls (list): list of fastq urls
    """
    lanes = sorted(lane_to_fastqs.keys())
    fastq_urls = []
    for lane in lanes:
        if read_index in lane_to_fastqs[lane]:
            manifest_entry = lane_to_fastqs[lane][read_index]
            fastq_urls.append(manifest_entry.url)
    return fastq_urls