while recv_num<tot_num:
    remain_num=tot_num-recv_num
    for i in range(512):
        data=receive_qword() # result=ser.read(size=8)
        f.write(data)
    recv_num = recv_num + 512

import array

while recv_num<tot_num:
remain_num=tot_num-recv_num
buffer = bytearray()
for i in range(512):
    buffer.extend(receive_qword()) # result=ser.read(size=8)
f.write(buffer) #yep, after the loop.
recv_num = recv_num + 512