serial port interfacing in python

Question

i have written a code in which i need to receive 3 bytes of code in which a receive a 2`s Complement Number of 24bits and i process the number , i am trying to receive the same in a list and try to initialise it after receiving a set of data here is the code i have written for the above

Received_Data=[] --- initialising the list 
while cnt<=18:    
  input_data = ser.read(3)
  Received_Data.append(input_data.encode("hex"))
  cnt = cnt+1
  cnt=0

after this my code continues for processing the 2`s complement received now the problem here is the if there is no data received the code should not come out the line input_data = ser.read(3) could anybody throw some light on the same if i have done it correctly or is there any other better way to code

Answer 1

You don't know how much data you will get from the port. Sometimes there will be data, but maybe more or less than exactly 3 bytes. I also don't quite understand your question, but I assume you don't want to process any commands until you get them, correct? And you want to get 18 commands of 3 bytes each stored in the list? Then may I suggest something like this?

received_data = [] --- initialising the list
cnt = 0
inbytes = ""
while cnt <= 18:
    while len(inbytes) < 3:
        inbytes += ser.read(3)
    while len(inbytes) >= 3:
        received_data.append(inbytes[:3].encode("hex"))
        inbytes = inbytes[3:]
        cnt += 1

I have not run this code myself, but I hope it helps you with the concept.