def METHOD_NAME(spec, start_slot, block_lengths):
    b = [spec.hash_tree_root(ByteList[spec.MAX_SHARD_BLOCK_SIZE](get_custody_test_vector(x)))
         for x in block_lengths]
    shard_transition = spec.ShardTransition(
        start_slot=start_slot,
        shard_block_lengths=block_lengths,
        shard_data_roots=b,
        shard_states=[spec.ShardState() for x in block_lengths],
        proposer_signature_aggregate=spec.BLSSignature(),
    )
    return shard_transition