pyserial - How to read the last line sent from a serial device

Question

I have an Arduino connected to my computer running a loop, sending a value over the serial port back to the computer every 100 ms.

I want to make a Python script that will read from the serial port only every few seconds, so I want it to just see the last thing sent from the Arduino.

How do you do this in Pyserial?

Here's the code I tried which does't work. It reads the lines sequentially.

import serial import time  ser = serial.Serial('com4',9600,timeout=1) while 1:     time.sleep(10)     print ser.readline() #How do I get the most recent line sent from the device? 

Answer 1

Perhaps I'm misunderstanding your question, but as it's a serial line, you'll have to read everything sent from the Arduino sequentially - it'll be buffered up in the Arduino until you read it.

If you want to have a status display which shows the latest thing sent - use a thread which incorporates the code in your question (minus the sleep), and keep the last complete line read as the latest line from the Arduino.

Update: mtasic's example code is quite good, but if the Arduino has sent a partial line when inWaiting() is called, you'll get a truncated line. Instead, what you want to do is to put the last complete line into last_received, and keep the partial line in buffer so that it can be appended to the next time round the loop. Something like this:

def receiving(ser):     global last_received      buffer_string = ''     while True:         buffer_string = buffer_string + ser.read(ser.inWaiting())         if '\n' in buffer_string:             lines = buffer_string.split('\n') # Guaranteed to have at least 2 entries             last_received = lines[-2]             #If the Arduino sends lots of empty lines, you'll lose the             #last filled line, so you could make the above statement conditional             #like so: if lines[-2]: last_received = lines[-2]             buffer_string = lines[-1] 

Regarding use of readline(): Here's what the Pyserial documentation has to say (slightly edited for clarity and with a mention to readlines()):

Be careful when using "readline". Do specify a timeout when opening the serial port, otherwise it could block forever if no newline character is received. Also note that "readlines()" only works with a timeout. It depends on having a timeout and interprets that as EOF (end of file).

which seems quite reasonable to me!