How to understand rdispls (recv_displacement) quantity in MPI_Alltoallv

Question

As titled, I think it should be a cumulated quantity from recv_count. So for the i process, the received data are inserted to the location recv_data + rdispls(i)*extent(recv_type). To make this clear, I try to rewrite the example in Python. It gives a wrong answer due to the wrong recv_disp (maybe as well the send_disp):

Example: https://rookiehpc.org/mpi/docs/mpi_alltoallv/index.html

My attempt in Python: (here I did not change the order recv_data in process 0 as that in the example)

from mpi4py import MPI
import numpy as np

comm = MPI.COMM_WORLD
size = comm.Get_size()
rank = comm.Get_rank()

assert size == 3

# Send data
if rank == 0:
    send_data = np.array([0, 100, 200],   dtype=np.int32)
    send_rank = np.array([0, 1,   1],     dtype=np.int32)
elif rank == 1:
    send_data = np.array([300, 400, 500], dtype=np.int32)
    send_rank = np.array([2,   2,   2],   dtype=np.int32)
else:
    send_data = np.array([600],           dtype=np.int32)
    send_rank = np.array([0],             dtype=np.int32)

send_length = len(send_data)
send_counts = np.zeros(size, dtype=np.int32)
for procs_id in send_rank:
    send_counts[procs_id] += 1

send_disp     = np.zeros(size, dtype=np.int32)
send_disp[1:] = np.cumsum(send_counts)[:-1]

# Receive data
recv_counts = np.zeros(size, dtype=np.int32)
comm.Alltoall(send_counts, recv_counts)
recv_length = np.sum(recv_counts)
recv_data = np.zeros(recv_length, dtype=np.int32)
recv_disp = np.zeros(size, dtype=np.int32)
recv_disp[1:] = np.cumsum(recv_counts)[:-1]         # Comment this and get correct answer

# Alltoallv
comm.Alltoallv([send_data, send_counts, recv_disp, MPI.INT],
               [recv_data, recv_counts, recv_disp, MPI.INT])

print(f"rank:\n{rank}\n"
      f"send_counts:\n{send_counts}\n"
      f"recv_counts:\n{recv_counts}\n"
      f"send_disp:\n{send_disp}\n"
      f"recv_disp:\n{recv_disp}\n"
      f"send_data:\n{send_data}\n"
      f"recv_data:\n{recv_data}\n"
      f"recv_length:\n{recv_length}\n"
      )