def read_bytes(filename, offset, number):
    """
    Reads specific bytes of a binary file.

    :param filename:    path to file
    :type filename:     string

    :param offset:      start reading at offset
    :type offset:       integer

    :param number:      number of bytes
    :type number:       integer

    :return:            byte string
    """
    with open(filename, "rb") as f:
        f.seek(offset)
        byte_string = f.read(number)
        f.close()
        return byte_string